Saturday's 2-1 loss was Bochum's fifth in six matches, and leaves the club 15th in the standings, three points above the relegation zone. In February, they defeated champions Bayern Munich 3-2, but have collected one point since then.
"We haven't managed to stabilise ourselves in the long term and make the last few games as successful as they could have been," Bochum sporting director Marc Lettau said in a statement.
"The team has the quality to survive in the Bundesliga, which is why we want to provide a decisive impetus for success through the move."
Letsch took over at Bochum in September 2022, with the club bottom of the Bundesliga on one point after seven games, and guided them to safety, finishing 14th last season.
"I will never forget this and it is not only this special experience that will always connect me with VfL, the city and the fantastic fans," Letsch said.
Bochum have six league games remaining and host mid-table Heidenheim on Saturday.
