Shams Charania of Yahoo Sports reported the news Saturday.
Len, 25, spent his first five seasons with the Phoenix Suns. He averaged 8.5 points and 7.5 rebounds in 2017-18.
The Hawks could be buying low on Len at a perfect time. He had his most efficient season of his career, setting a career high in field-goal percentage (56.6), rebound rate (19.8), assist rate (9.1) and win shares (4.5), per Basketball Reference.
The marriage between Len and the Suns seemed destined to end all season. He admitted in March that he was probably leaving Phoenix in free agency.
"I'm looking forward to this offseason," Len told Scott Bordow of AZCentral. "I think it's going to be exciting. It's the first time I'm actually going to have a chance to go where I want to go."
Len will likely compete with Dewayne Dedmon and Miles Plumlee for minutes at center. Dedmon signed with the Hawks last offseason and started 46 games in 2017-18.