She added: "Thank you @sagaftra negotiators and leadership for getting us over the finish line!!! Gratitude is the attitude!!"
Alec Baldwin offered his "congratulations to everyone who did this great work on behalf of the members", in a post on Instagram.
Sag-Aftra chief negotiator Duncan Crabtree-Ireland told Reuters there were "definitely some tears, a lot of big smiles, a lot of hugs" when the agreement was reached.
The three-year contract would "make a long-term difference for the future of our members in this industry", he said.
The union said the deal was valued at more than $1bn (£814m) and included increases in minimum salaries, a new "streaming participation" bonus, and more protections against their images and voices being replicated by artificial intelligence.
Sag-Aftra said the strike would officially end on Thursday, with more details released following a meeting on Friday.
Union negotiating committee member Kevin E West told Variety that there were "tears of exhilaration and joy" after the contract was approved, but that the agreement was "not perfect".
Fellow committee member Shaan Sharma told the New York Times he had mixed emotions because not all of the union's demands were met.
"You can be happy for the deal overall, but you can feel a sense of loss for something that you didn't get that you thought was important," he said.
AMPTP said it was pleased to have reached the tentative agreement and "looks forward to the industry resuming the work of telling great stories".
It said the deal gave Sag-Aftra "the biggest contract-on-contract gains in the history of the union".
In addition to increased pay and AI guarantees, Sag-Aftra has called for increased royalties and higher contributions to actors' pension and health plans.
Sag-Aftra represents about 160,000 members and has been on strike since July 14, causing major disruption and knock-on effects for those in all branches of the film and TV industry, and in countries like the UK as well as the US.
Disney/Marvel's Blade, Dune: Part Two and Fantastic Four have all been delayed by several months, while Avengers: The Kang Dynasty and Avengers: Secret Wars have also been pushed back by a year.
Live action remakes of Disney animations Moana and Lilo & Stitch have also been affected, as has James Cameron's Avatar series and Paddington in Peru.
As well as film delays, Hollywood stars have also not been attending some events such as premieres while the strike has been taking place, as union rules prohibit them from taking any work, including promotion or publicity for projects.
Some productions, like The Iron Claw, had an "interim agreement", meaning stars could do interviews and attend the premiere.
Actors had also not been allowed to attend awards ceremonies, meaning the Emmys - TV's biggest night - were delayed from their usual September slot.
The Emmys ceremony will take place in January instead, and organisers of events like the Oscars are likely to be breathing a sigh of relief that stars can hit the red carpet when the movie awards season kicks off in the New Year.