The union wants streaming companies to agree to a more equitable profit share and improved working conditions.
The strike means that 160,000 actors and actresses will cease working at midnight.
According to director Christopher Nolan, stars Cillian Murphy and Emily Blunt fled the Oppenheimer premiere as the strike began.
The SAG strike will begin at midnight (08:00 BST) in Los Angeles. An hour later, picking will begin outside Netflix’s California offices, before continuing on to Paramount, Warner Bros., and Disney.
The union also wants assurances that no artificial intelligence or computer-generated looks or voices would be utilized to replace performers.
According to a strike order posted online by SAG, the walkout includes people engaged in acting, singing, or dancing.
He added that the strike “is an instrument of last resort”.
The group representing the studios, the Alliance of Motion Picture and Television Producers, or AMPTP, said that “a strike is certainly not the outcome we hoped for as studios cannot operate without the performers that bring our TV shows and films to life”.
“The union has regrettably chosen a path that will lead to financial hardship for countless thousands of people who depend on the industry,” its statement added.
Fran Drescher, SAG’s president, said the strike comes at a “very seminal moment” for actors working in the industry.
“What’s happening to us is happening across all fields of labour,” she said, “when employers make Wall Street and greed their priority, and they forget about the essential contributors that make the machine run.”