Riley closed with a 2-under 70, while Lovemark fired a 7-under 65 as they finished at 16-under-par 272.
At the par-5 18th, the first playoff hole, both players made par and moved to the par-3 17th.
Riley made par for the win as Lovemark stumbled to a bogey to lose in just his second PGA TOUR-sanctioned start.
'I wasn't thinking I had to shoot a certain number today,' said Riley, who collected $90,000 for the win. 'I knew it was going to be tough. Jamie played some great golf today. It feels great to win again. This is not an easy tour to win on.'
Esteban Toledo, who shared the third-round lead with Jay Delsing, stumbled to a 1-over 73 in the final round. He fell into a share of third at 14-under-par 274, where he ended alongside Greg Chalmers (72).
Delsing, who also led after round two, faltered to a 2-over 74. He was joined at minus-13 by Patrick Sheehan, Richard Johnson, Lee Williamson, Jeremy Anderson and Gary Christian.
Lovemark, the reigning NCAA champion who played on a sponsor's invite this week, flew out of the gate with three birdies over the first four holes. He made the turn at 13 under after a birdie on the eighth at Somerby Golf Club.
The freshman at Southern California, who teed off well ahead of the leaders, birdied the 11th. Lovemark drained back-to-back birdies from the 14th to get to 16 under.
Lovemark, who started the round tied for 24th, still trailed by one at that point, but as he parred the final three holes the leaders fell back to him. He posted one of just two bogey-free rounds on the day.
So, Lovemark waited in the clubhouse as Riley finished the final eight holes. He would have been the first amateur to win on the Nationwide Tour.
'I didn't know what to expect,' said Lovemark, who matched Charles Howell III for the best finish by an amateur in tour history. 'This is the second-best tour in the world and I learned a lot from playing with guys like this.'
Riley opened with a birdie on the first, but gave that stroke back with a bogey at the par-4 third. Birdies at seven and 11 gave him a share of the lead at 16 under.
The former U.S. Ryder Cupper tripped to a bogey at the par-3 13th. Riley closed with a two-putt birdie, from 99 feet out, on the par-5 18th to force the extra session.
'The story of my week was my putter,' admitted Riley. 'I wasn't making many putts so I went into my closed and pulled out an old Ping putter. I made everything I had to this week, so that putter will stay with me for a while.'
Josh Broadaway and Fabian Gomez each closet with 68s. They were joined in a share of 11th at 12-under-par 276 by Jason Enloe, Chris Anderson, Chez Reavie and Tommy Tolles.