The governor's office also released a 41-page slide-show presentation detailing its reasons for accepting the expansion.
Sen. Joe Manchin (D-W.Va.), a conservative Democrat elected after the healthcare law passed, said he supports Tomblin's decision.
"I respect and support his decision to go ahead with expansion in a way that is tailored to West Virginia," Manchin said. "I will do everything I can at the federal level to make sure he has all the flexibility he needs to make this program work in a fiscally responsible way for the benefit of the West Virginians it is intended to help."
The expansion will cover more than 91,000 people in West Virginia, according to Tomblin's office.
The Affordable Care Act expands Medicaid eligibility to people with incomes up to 133 percent of the federal poverty level.
It was initially expected to account for half of the health law's total coverage expansion, before the Supreme Court ruled that states must be able to opt out of the expansion without quitting Medicaid altogether.
Conservative activists have pressed GOP governors to resist the expansion, but several high-profile governors including Chris Christie (N.J.), Jan Brewer (Ariz.), Rick Scott (Fla.) and John Kasich (Ohio) have signed on.
Sen. Jay Rockefeller (D-W.Va.) also praised Tomblin, saying the Medicaid expansion would deliver real help to the 1 in 4 West Virginians who are uninsured.
"Gov. Tomblin's decision is based on the facts, and it's a good decision for West Virginia families," Rockefeller said.