The pop star branded the US state’s decision, which was authorised on Wednesday, a “travesty” and said women and young girls would suffer at the hands of the new system.
Alabama's law sparked a fierce and immediate backlash from opponents, with many famous faces speaking out against it including Courteney Cox and Reese Witherspoon.
Joining with words of condemnation, Gaga wrote on Twitter: “It is an outrage to ban abortion in Alabama, period, and all the more heinous that it excludes those who have been raped or are experiencing incest, non-consensual or not.
"So there's a higher penalty for doctors who perform these operations than for most rapists? This is a travesty, and I pray for all these women and young girls who suffer at the hands of this system."
— Lady Gaga (@ladygaga)May 15, 2019
Amid furious opposition from women's rights advocates, the US state's Senate voted 25-6 for the bill before it was signed off by Republican governor Kay Ivey.
The bill, set to come into effect in six months, will block abortions in the event of rape and incest, with the only exception being when a woman's health is at serious risk.
Doctors who defy the law could face 99 years in prison, the maximum sentence for a Class A felony.
Friends star Cox was born in the Alabama state capital, Birmingham, and said she was "appalled" by the law. She said: "I love Alabama and I am proud to call it my home. But today I am appalled and scared. We CANNOT go backwards!"
Cox also urged fans to donate to an Alabama-based pro-abortion charity.
Oscar-winning actress Reese Witherspoon tweeted her dismay, while also condemning similar laws in other states. She said: "I'm beyond upset about the passing of new abortion bans in Alabama, Kentucky, Mississippi, Georgia, and Ohio. This is Unconstitutional and Abhorrent. We cannot tolerate this attack on women's fundamental rights.
Earlier, British actress Jameela Jamil blasted the law as "truly disgusting". She had earlier revealed she once underwent an abortion and described it as the "best decision" she ever made.
Other celebrities speaking out included Captain America star Chris Evans, filmmaker Ava DuVernay and actress Alyssa Milano.
The introduction of the law has reignited the incendiary debate around abortion and raised the prospect of the conservative Supreme Court reviewing the landmark 1973 Roe v Wade decision legalising terminations nationally.