Fitch, a four-term member for the riding of Riverview, will also be leader of the official Opposition when the legislature resumes in December.
Party president Jason Stephen made the announcement Saturday following a meeting of the presidents of the 49 riding associations.
"We're excited to have Mr. Fitch and his experience to hold the Liberal government to account on the house side, and we're certainly doing everything we can as a party to make ourselves election ready for 2018," he said.
No date has been chosen for a convention to choose a permanent leader.
Fitch is a former mayor of Riverview and was first elected to the legislature in 2003.
He thanked Alward for his work as leader and said the party's priority now is the byelection in the riding of Saint John East on Nov. 17.
Party president Jason Stephen said Glen Savoie will again be their candidate.
Savoie lost by nine votes to Liberal Gary Keating in last month's provincial election, but Keating quit this week citing concerns for his health and family.