The duo take over from Peter Reid who was sacked by the club on Sunday after taking just one point from nine matches.
Both Fletcher and Larrieu are already on Plymouth’s playing staff.
“I can assure all of our supporters that I will give nothing short of 100% in attempting to turn round our fortunes on the field,” Fletcher said in a statement.
“I believe that, with the current squad and perhaps one or two additions, there is no reason why we cannot return to winning ways sooner rather than later.
“Our supporters have been magnificent during a turbulent period in our history and I am sure that, with their continued help, we can quickly put the past behind us and start to climb back up the Football League.”