Numerous bold-faced names posted their reactions to JB stepping down this morning -- and, while some thanked the prez for his service, others are slamming his announcement and calling on him to resign

Barbra Streisand came out quickly after the news ... thanking Biden for his significant achievements in his one term as president -- while Cardi B gloated about how she called this happening weeks ago.

Kathy Griffin, Mark Hamill, Billy Eichner and Rob Reiner -- the latter of whom actually called on Biden to back down earlier this month -- all tweeted messages of support for the Democratic party with some tributing Biden.

In a statement to TMZ, Robert De Niro says, in an act of patriotism, "Joe Biden is stepping aside to clear the path for another Democrat to become president because there is nothing more important for our country than defeating Donald Trump at the ballot box. With respect, admiration, and affection, thank you Mr. President!”

Other celebs are voicing their displeasure ... including conservative commentator Piers Morgan -- who's echoing Republican demands that Biden resign the presidency if he can't run again.

Barstool Sports founder Dave Portnoy said he thinks it's all a conspiracy ... adding there's no way Kamala Harris -- who Biden endorsed -- would be the nominee in an open primary field, and special interest groups control her just like Biden.

Roseanne Barr, Russell Brand and James Woods are just a few of the more conservative Hollywood elites who are putting the prez on blast BTW.

Ya gotta imagine most of H'wood's happy with the news though ... given the number of stars who have asked him to step down since George Clooney's New York Times op-ed came out earlier this month.