Theriault says he decided not to seek re-election because of advice from his doctor.
Get the stress out of my life — those were the doctor’s orders,” Theriault said in a statement on Friday.
“And if you’re going to do this job right, it has to be stressful.”
Theriault spent 35 years in the commercial fishing industry in southwestern Nova Scotia before he got into politics.
He first won the province's Digby-Annapolis seat in 2003.
He is the Liberal party's fisheries critic and has also served as the critic for seniors, communities and culture and heritage.
Known for his affability and his ability to turn a colourful turn of phrase, Theriault drew praise from Premier Darrell Dexter.
"He has a great sense of humour and has always worked hard to represent the people of Digby-Annapolis," said Dexter. "I know how demanding public service can be and Junior always took the role seriously."
Progressive Conservative Leader Jamie Baillie called Theriault "one of a kind."
"His presence will be missed, including his unique voice and down to earth sense of humour," Baillie said.
The NDP was elected to a majority government three years ago and doesn't have to call an election until June 2014.