The rules for improvised weapons state that you can throw a melee weapon even if it does not have the thrown property for 1d4 damage:

If a character uses a ranged weapon to make a melee attack, or throws a melee weapon that does not have the thrown property, it also deals 1d4 damage.

It is also stated that using an improvised weapon when it is similar to an existing weapon allows one to add their proficiency bonus to the attack:

At the GM’s option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus,

Does this mean that when throwing a weapon this way, if a character is proficient with the weapon being thrown they get to add their proficiency bonus to this attack?

(The character does not have the Tavern Brawler feat which would make this trivial. This question asked in regards to a discussion on this homebrew question)

Only if you are ruled as using an improvised version of a proficient weapon

The relevant piece of information, PHB p. 147:

Improvised Weapons

In many cases, an improvised weapon is similar to an actual weapon and
can be treated as such. For example, a table leg is akin to a club. At
the DM’s option, a character proficient with a weapon can use a
similar object as if it were that weapon and use his or her
proficiency bonus.

An object that bears no resemblance to a weapon deals 1d4 damage (the
DM assigns a damage type appropriate to the object). If a character
uses a ranged weapon to make a melee attack, or throws a melee weapon
that does not have the thrown property, it also deals 1d4 damage. An
improvised thrown weapon has a normal range of 20 feet and a long
range of 60 feet.

If you use an object that's similar to a weapon as if it were that weapon, that assumes you're using the weapon it was intended, with all of the applicable damage types, proficiencies, and properties associated with it.

Alternatively, if you use an object that bears no resemblance to a weapon, the object can deal 1d4 damage and the Thrown property.

However, you cannot use an object that's similar to a weapon and use it as if it were that weapon, and still have the object be something that bears no resemblance to a weapon.

If you attempt to throw a weapon without the thrown feature, you are no longer using the weapon the way it was intended, and it has to be treated as a different weapon. If a different weapon is applicable, use the new weapon's properties and proficiencies. Otherwise, treat it as an Improvised Weapon, with the applicable properties and proficiency.

With this information, you effectively have two choices:

1. Have your DM treat the original weapon as a second, different weapon that has the Thrown feature. Example: treating a Halfling's War Hammer (no Thrown) as a Goliath's Light Hammer (with Thrown).




2. Make your attack as using an Improvised Weapon, dealing 1d4 damage and using the default thrown option of 20/60 range, and only adding proficiency if provided by another feature (as from Kensei or Tavern Brawler)

Probably yes, but at the DM’s option

First, I assume the DM rules that some weapon you are proficient in is “similar” to a weapon your “character is proficient with.” I have a hard time imagining how anyone could argue that this is not the case, since to do so would require claiming that the weapon is not an object “similar” to itself. But at any rate, the relevant rule is explicitly “at the DM’s option.”

Second, there is a grammar ambiguity in that rule, about what exactly the word and means in a particular place, and the interpretation of that and directly impacts whether or not you get proficiency. Since this is in the rule about the DM’s options here, I’m calling the ruling on that ambiguity to also be part of the DM’s option.

For what it’s worth, as DM I absolutely would allow proficiency here. Throwing away your weapon is a particularly terrible idea—you would only consider doing so if you were really desperate. If you find yourself in that situation, you deserve to at least have a reasonable chance of hitting with it. And I think it matches the rules text better.

Relevant rules quotes:

If a character [...] throws a melee weapon that does not have the thrown property, it also deals 1d4 damage. An improvised thrown weapon has a normal range of 20 feet and a long range of 60 feet.

This shows that when you throw a non-thrown-melee weapon, it is an “improvised thrown weapon.”

In many cases, an improvised weapon is similar to an actual weapon and can be treated as such. For example, a table leg is akin to a club.

This is a possibility for any improvised weapon. A non-thrown-melee weapon that gets thrown counts as a improvised thrown weapon, which is to say it is an improvised weapon, and since it actually is a weapon, it is certainly “similar” to the weapon it actually is.

At the DM’s option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus.

And this shows that in the case of an improvised weapon being similar to an actual weapon, which almost-certainly must be the case here, proficiency bonus applies.

Except, and this is an important bit, we have “[you] can use [it] as if it were that weapon and use [your] proficiency bonus,” here. That and is ambiguous: either this rule lists two separate things you “can” do, and they are independent, or this is listing one thing you can do, which consists of both parts together. That is, which of these is the correct parsing of the sentence?

• 
  

  [you] can

  
  
  

  
  
  • use [it] as if it were that weapon
  
  

  
  

  and

  
  
  

  
  
  • use your proficiency bonus,
  
  

  
  




• 
  

  [you] can

  
  
  

  
  
  • use [it] as if it were that weapon and use [your] proficiency bonus
  
  

  
  

The repeat of the verb use here points to these two clauses being two separate entries: you can use it as if it were that weapon, and also, you can use your proficiency bonus with it. But that is an exceedingly high level of legalistic specificity to expect of a D&D rule; we don’t generally assume that the rules are written that carefully. Which means that this is, arguably, ambiguous, and you (or more relevantly, the DM) can decide it means them as one thing you can do, and so you only get the proficiency bonus if you use the weapon-like object as that weapon. Throwing a greatsword would not be that.

(The other argument you could make is that, say, a greataxe is similar to a handaxe, which can be thrown, so if you throw a greataxe you are using it as if it were a handaxe, so your handaxe proficiency should apply. Also a plausible ruling for a DM to make. Certainly, if you bash someone with a bowstaff, arguing that it is similar to a club or quarterstaff seems quite reasonable.)

Irrelevant rules quotes

Just because there is other rules text here that seems to be confusing some folks, these are some rules that don’t actually apply to this situation.

An object that bears no resemblance to a weapon deals 1d4 damage (the DM assigns a damage type appropriate to the object).

A weapon is not an object that bears no resemblance to a weapon. Therefore this sentence, in its entirety, does not apply, which is precisely why the next line, quoted above, says that throwing non-thrown-melee weapons “also” reduces their damage to 1d4.

If you use a weapon in a way that turns it into an improvised weapon—such as smacking someone with a bow—that weapon has none of its regular properties, unless the DM rules otherwise.

^{(Jeremy Crawford tweet)}

We aren’t discussing a regular property of the weapon—by definition we are discussing a property the weapon doesn’t regularly have, since we are talking about the thrown property of a weapon that doesn’t have it—so the fact that a weapon doesn’t have its regular properties is irrelevant. Jeremy Crawford does not deny an improvised thrown weapon its irregular thrown property, which the rules explicitly grant it.

Yes

The rules for weapon proficiency state (emphasis mine):

Proficiency with a weapon allows you to add your proficiency bonus to the Attack roll for any Attack you make with that weapon.

Since throwing a weapon you are proficient with is making an Attack roll with that weapon, you may add your proficiency bonus to the Attack roll.

To cite the rules cited in Daniel Zastoupil's answer:

In many cases, an improvised weapon is similar to an actual weapon and can be treated as such. For example, a table leg is akin to a club. At the DM’s option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her proficiency bonus.

An object that bears no resemblance to a weapon deals 1d4 damage (the DM assigns a damage type appropriate to the object). If a character uses a ranged weapon to make a melee attack, or throws a melee weapon that does not have the thrown property, it also deals 1d4 damage. An improvised thrown weapon has a normal range of 20 feet and a long range of 60 feet.

The bolded text makes it clear that using a ranged weapon for a melee attack or a melee weapon for a ranged attack does not cause that weapon to no longer be considered a weapon. When throwing a longsword at someone, you are still considered to be using a longsword. Using a weapon for an improvised purpose does not mean you should lose proficiency in it. Improvised weapons are weapons that were not considered weapons to begin with.

The damage reduction makes sense here as well - if you were to throw a sword designed for melee, it would stand to reason that the damage would not be as effective. As well, if hitting someone with a longbow, it stands to reason you would only deal 1d4 damage rather than the regular 1d8, since the 1d8 comes from the proper use of the weapon to fire an arrow.