Ralston, the member for Surrey-Whalley, will take over house duties immediately from Horgan, who announced on Monday that he's running to replace Leader Adriane Dix.
Horgan, the NDP's energy critic, said he decided to enter the leadership race after other New Democrats persuaded him to run.
Horgan and NDP finance critic Mike Farnworth are the only contenders so far in the leadership bid.
Dix announced his resignation last fall, following the New Democrat's unexpected loss to the Liberals in the May 2013 provincial election.
The leadership vote is slated for September in Vancouver.
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