It happened at about 01:00 BST between junction 8 southbound for the M50 and junction 9 for Tewkesbury.
Police and paramedics attended. Two people were pronounced dead at the scene.
The M5 is closed between J8 and J9 southbound, and officers are advising motorists to avoid the area.
A diversion has been put in place to exit the M5 at Junction 8 and follow signs via the A417 and A40.
Police had earlier said that three people were dead, but later corrected this information.
Two people died and a third person was seriously hurt in an accident involving at least four vehicles on the M5.