Morales, 35, is expected to play first base while Gold Glover Matt Olson recovers from a broken hamate bone in his right hand. Olson was injured Thursday in the second game of the team's season-opening series against the Seattle Mariners in Japan. He had surgery the following day, and the team didn't provide a timeline for his return, although he is expected to be out for at least a month or two.
The switch-hitting Morales hit .249 with 21 homers and 57 RBIs in 130 games last season for Toronto, his second with the Blue Jays, and he will help balance out the A's lineup.
"Kendrys has consistently mauled right-handed pitching throughout his career," A's general manager David Forst said, according to the San Francisco Chronicle. "There's no doubt in our minds he can continue to do it."
Morales is in the final year of a three-year, $33 million contract. Toronto is responsible for more than $10 million of the $12 million left on the deal, the Chronicle reported.
The Blue Jays acquired minor league infielder Jesus Lopez and international signing bonus pool space in the deal. In addition to Morales, the A's received cash considerations.
The A's also transferred right-hander Jharel Cotton to the 60-day injured list to make room on the 40-man roster for Morales.
The Associated Press contributed to this report.