Griffiths comes to Wellington after a major falling out at Newcastle Jets, where he was axed by club owner Nathan Tinkler late last month along with skipper Kew Jaliens (now at Melbourne City), David Carney, Adrian Madaschi and Billy Celeski.
Merrick says he's looking forward to working with Griffiths.
"Joel has a great work ethic and has scored goals wherever he has played," Merrick said.
"He's just the type of player we need with the season entering a crucial stage."
Griffiths, 35, is enthusiastic about what lies ahead in Wellington.
"I'm looking forward to this next chapter in my career and I have a really good feeling about it," he said.
"I've been watching the Phoenix this season and it isn't the team I used to know.
"They're playing really good football and they have some X-factor players in the team.
"They have two of the most dangerous players in the league up front, have a talented midfield, and at the back have guys who have played together for a long time and are comfortable with each other."
