The legendary singer -- who's been a staple in the genre for decades -- just posted a sweet tribute to Bey with a congratulations message over her hitting #1 on Billboard's Hot Country chart ... and she seems to be extending even more of a cosign here too.

She writes, "I'm a big fan of Beyoncé and very excited that she's done a country album. So congratulations on your Billboard Hot County number one single. Can't wait to hear the full album!" Dolly captioned her post with a heart for good measure ... yeah, it's all love.

If there was ever someone in country you could want to hear from on this issue -- it's Dolly ... who's cemented her legacy, and who most certainly can speak on the community at large.

The fact she's out here singing Bey's praises speaks volumes ... and should quiet the haters.

Just about every major country star has been enthused about Beyoncé coming into the fold with them ... and even other Black country musicians have spoken out about this too.

Darius Rucker just talked to a photog the other day and said he also couldn't wait to hear what Beyoncé had in store for 'Act II' ... and he even went so far as to say he hoped it snagged her the long-elusive Album of the Year Grammy that she hasn't been able to lock down yet.

As for Bey ... well, she's let the music do the talking -- everyone seems to be loving "Texas Hold 'Em" and "16 Carriages" ... especially the former, which is the song that's #1 right now.

There was some controversy when it first dropped last week ... as some felt certain radio stations were going out of their way to keep Beyoncé's tune out of the rotation on the air.

Perhaps that was happening at some stations ... but others appeared to be unfairly targeted.