Atem, 23, arrives on the recommendation of Adelaide coach Joey Wright.

"We had Atem down for a couple of days last week and while we were impressed with him he was just as impressed with us and was willing to sign," head coach Phil Thomas said.

"It is good to get a player who is just embarking on his NBL career and I think he will be capable of turning a few heads for the Thunder."

Thomas also announced that versatile American Garrett Jackson would return in 2016.

Jackson, 24, made a huge impact when he joined in mid-season and has since signed to play with Melbourne United in the NBL.

Many other SEABL clubs were seeking Jackson's service, but the Thunder have been able to retain him.

"It is good news for the Thunder to have Garrett back along with the signing of Atem," Thomas said.

"Garrett is starting to adapt to what is required at NBL level and for people who know him he's such a hard working player."

Atem was offered a contract after enjoying two days of the Thunder's hospitality last week and impressed Thomas in a brief workout.

The Thunder beat premiership team Mount Gambier and the Albury-Wodonga Bandits in the bid to sign him.

Atem joins as a local import and replaces Sydney's Nathan Wilson who was released at the end of the season.

This still leaves the Thunder with the opening for a second US import to replace Texan Fred Hunter.

Hunter won Thunder's most valuable player award this season, but it seems highly unlikely he will return in 2016.

Thomas said he was looking forward to the Thunder improving on this season's third place finish in the SEABL South Conference.

"We've got some good depth and are now ready to improve," he said.