Boa tarde, gostaria de saber se é possível realizar um ataque com escudo usado como arma improvisada sem perder AC.

Yes, attacking with a shield doesn't make you loose the +2 AC. 

Good question. The rules don't make a clear yes or no statement about that. It would be up to the DM (Dungeon Master).

There is nothing in the rules that says you cannot.

Quote from IamSposta >>

There is nothing in the rules that says you cannot.

 That is generally not how rules work. They say what you can do, if they don't say you can do that, you probably can't. "Improvise" actions covers a lot, but is up to DM.

Improvised weapons are similarly vague, but say the object must be wielded in 1 or 2 hands. Shields have to be donned to confer their AC benefit. It is up to a DM if something (presumably) strapped to your arm counts as being "in" your hand.

I think most DMs would allow it, but I don't think it counts as RAW either way.

Quote from DxJxC >>

Quote from IamSposta >>

There is nothing in the rules that says you cannot.

 That is generally not how rules work. They say what you can do, if they don't say you can do that, you probably can't. "Improvise" actions covers a lot, but is up to DM.

 

 
Okay, allow me to rephrase:

There is nothing in the rules for improvised weapons the expressly prohibits the improvised weapon from being a shield.

Quote from Elconfessor >>

Boa tarde, gostaria de saber se é possível realizar um ataque com escudo usado como arma improvisada sem perder AC.

Hello Elconfessor,

Yes. The section in the Basic Rules on Improvised Weapons states that anything can be a weapon. Further, shields confer their benefit so long as you are holding it in your hand. Therefore, you can use the shield as an improvised weapon and maintain your increased AC so long as you are wielding the shield.

Olá Elconfessor,
Sim. A seção das Regras Básicas sobre Armas Improvisadas afirma que qualquer coisa pode ser uma arma. Além disso, os escudos conferem seus benefícios enquanto você os estiver segurando em suas mãos. Portanto, você pode usar o escudo como uma arma improvisada e manter sua classe de armadura aumentada enquanto estiver empunhando o escudo.

Quote from IamSposta >>

Quote from DxJxC >>

Quote from IamSposta >>

There is nothing in the rules that says you cannot.

 That is generally not how rules work. They say what you can do, if they don't say you can do that, you probably can't. "Improvise" actions covers a lot, but is up to DM.

 

 
Okay, allow me to rephrase:

There is nothing in the rules for improvised weapons the expressly prohibits the improvised weapon from being a shield.

Interesting topic.

Nothing prevents this you say. But isn't there? My initial gut instinct is "of course there has to be". Surely something says something about the shield needing to be using a free hand, or something to that effect. But what do rules actually say when we review them?

A shield is made from wood or metal and is carried in one hand. Wielding a shield increases your Armor Class by 2. You can benefit from only one shield at a time.

This says a shield is carried in one hand, but this is descriptive.  It doesn't say anything about the hand needing to be free. So far we're good to go.

Sometimes characters don’t have their weapons and have to attack with whatever is at hand. An improvised weapon includes any object you can wield in one or two hands, such as broken glass, a table leg, a frying pan, a wagon wheel, or a dead goblin.

Here too, we are asked merely that something be at hand, and, a shield certainly is. No requirements listed for the hand being free, only that the object be in hand and wield-able, both things the shield is.

That brings us tot he final area that might give a restriction, the donning rules:

The time it takes to don or doff a type of armor or a shield is shown in the Donning and Doffing Armor table.

Don. This is the time it takes to put on the item. You benefit from its AC only if you take the full time to don it.

Doff. This is the time it takes to take off the item. If you have help removing armor, reduce this time by half.

Here it tells us we get the AC while the shield is donned. And, as we saw earlier, the shield needn't be doffed to use it as an improvised weapon, nor does it explicitly required to be free for either cases.

So in the end, we have permission to use it as an improvised weapon, and we didn't lose AC while doing so since it remained donned the whole while.

Not the result I figured initially, but is the result the rules tell us. Sposta is correct here.

It's worth mentioning that there's also an official ruling in Sage Advice concerning this very question;

If you attack with a shield—most likely as an improvised weapon—do you keep the +2 bonus to AC? Attacking with a shield doesn’t deprive you of the shield’s bonus to AC.

Quote from DxJxC >>

Quote from IamSposta >>

There is nothing in the rules that says you cannot.

 That is generally not how rules work. They say what you can do, if they don't say you can do that, you probably can't. "Improvise" actions covers a lot, but is up to DM.

Improvised weapons are similarly vague, but say the object must be wielded in 1 or 2 hands. Shields have to be donned to confer their AC benefit. It is up to a DM if something (presumably) strapped to your arm counts as being "in" your hand.

I think most DMs would allow it, but I don't think it counts as RAW either way.

 Actually it often is how they work. There are two “schools”  of rules and their interpretation: 

1) rules can define what’s allowed - with the assumption that anything not allowed is forbidden.
2) rules can forbids specific things —with the assumption that anything not forbidden is allowed.

Many of the D&D rules are written in a sufficiently ambiguous way that which of these was intended is unclear leading to many of the extended discussions here between those of one school and those of the other. In most such cases the real answer is - speak to your DM and see what they think the rule interpretation should be.

Well, from what I understand from the text, she doesn't prohibit this action, but many masters can't see this situation... Because they think it will break the game. I'll give you a special case now:

Using the shield as an improvised weapon, would it be possible to perform the attack in the bonus action? Using two-weapon combat rule... See combat would look like this: Action: attack with shield and in the bonus action attack with light weapon. It would be possible?

Quote from Elconfessor >>

Well, from what I understand from the text, she doesn't prohibit this action, but many masters can't see this situation... Because they think it will break the game. I'll give you a special case now:

Using the shield as an improvised weapon, would it be possible to perform the attack in the bonus action? Using two-weapon combat rule... See combat would look like this: Action: attack with shield and in the bonus action attack with light weapon. It would be possible?

 
No, because a shield does not have the light property. That is, unless the PC also had the Dual Wielder feat.

Quote from IamSposta >>

Quote from Elconfessor >>

Well, from what I understand from the text, she doesn't prohibit this action, but many masters can't see this situation... Because they think it will break the game. I'll give you a special case now:

Using the shield as an improvised weapon, would it be possible to perform the attack in the bonus action? Using two-weapon combat rule... See combat would look like this: Action: attack with shield and in the bonus action attack with light weapon. It would be possible?

 
No, because a shield does not have the light property. That is, unless the PC also had the Dual Wielder feat.

I think you mean "No, unless the shield has the light property." There aren't any that i know of in official content, or anything, sure. but if there is a shield with the light property in your games, you absolutely could TWFing with it.

Up to DM, Two-Weapon Fighting & Dual Welder is intended to work with melee weapon, not improvised weapon as per the Dev

@AdamInvests Can improvised weapons qualify as one handed weapons for the purposes of the Dual Wielder feat?

@JeremyECrawford Dual Wielder is intended to work with actual weapons. Using the feat with improvised weapons is up to the DM.

@YetiMoose A shield as an improvised weapon with the Dual Wielding feat: Does the feat's +1 AC stack with the shield +2 AC that round?

@JeremyECrawford Dual Wielder is meant to work (RAI) with a melee weapon or an equivalent, not something like a shield.

@YetiMoose Um, you're the one who previously Tweeted that a shield can be used as an Improvised Weapon! How's that not a melee weapon?

@JeremyECrawford Yes, a shield, like many things, can be used as an improvised weapon. This has no bearing on the design intent of Dual Wielder.

Quote from Ravnodaus >>

Quote from IamSposta >>

Quote from Elconfessor >>

Well, from what I understand from the text, she doesn't prohibit this action, but many masters can't see this situation... Because they think it will break the game. I'll give you a special case now:

Using the shield as an improvised weapon, would it be possible to perform the attack in the bonus action? Using two-weapon combat rule... See combat would look like this: Action: attack with shield and in the bonus action attack with light weapon. It would be possible?

 
No, because a shield does not have the light property. That is, unless the PC also had the Dual Wielder feat.

I think you mean "No, unless the shield has the light property." There aren't any that i know of in official content, or anything, sure. but if there is a shield with the light property in your games, you absolutely could TWFing with it.

 That would be insufficient, for the same reason you can't TWF with a hand crossbow.  Improvised weapons do not gain the ranged or melee... I'll refer to that as "category" from being improvised.  So:

• If you club someone with a longbow, it does not become a melee weapon.  It does remain a ranged weapon.
• If you throw a warhammer at someone, it does not become a ranged weapon.  It does remain a melee weapon.
• If you bash with a shield, it does not become a melee weapon or a ranged weapon.  It remains neither.

Quote from quindraco >>

Quote from Ravnodaus >>

Quote from IamSposta >>

Quote from Elconfessor >>

Well, from what I understand from the text, she doesn't prohibit this action, but many masters can't see this situation... Because they think it will break the game. I'll give you a special case now:

Using the shield as an improvised weapon, would it be possible to perform the attack in the bonus action? Using two-weapon combat rule... See combat would look like this: Action: attack with shield and in the bonus action attack with light weapon. It would be possible?

 
No, because a shield does not have the light property. That is, unless the PC also had the Dual Wielder feat.

I think you mean "No, unless the shield has the light property." There aren't any that i know of in official content, or anything, sure. but if there is a shield with the light property in your games, you absolutely could TWFing with it.

 That would be insufficient, for the same reason you can't TWF with a hand crossbow.  Improvised weapons do not gain the ranged or melee... I'll refer to that as "category" from being improvised.  So:

• If you club someone with a longbow, it does not become a melee weapon.  It does remain a ranged weapon.
• If you throw a warhammer at someone, it does not become a ranged weapon.  It does remain a melee weapon.
• If you bash with a shield, it does not become a melee weapon or a ranged weapon.  It remains neither.

That's actually up to the DM per the improvised damage rules.

"Often, an improvised weapon is similar to an actual weapon and can be treated as such."

If you had a shield that was similar enough to a dagger, for example, you could use it as one. Obviously this is all subjective so DM buy-in required. But still RAW.