Patrick says Cowan's role in helping turn around the Massachusetts economy in recent years "has earned him the respect and admiration of people throughout government." He says the state will benefit from Cowan's wisdom and good judgment.
Cowan, a North Carolina native, says he is "honored and humbled" by the appointment.
Cowan is the former chief of staff and chief legal counsel to Patrick. He served in the administration since 2009.
Cowan will serve as senator until voters elect a new senator during a special election on June 25.
Kerry's resignation from the Senate takes effect Friday.
Associated Press writers Steve Peoples and Steve LeBlanc contributed to this report.