Boyd fired a 6-under 66 on Saturday, while Barcelo only managed a 3-under 69. The pair is knotted at 14-under-par 274, which is good for a one-shot lead.
George McNeill posted a 3-under 69 and is alone in third place at 13-under-par 275.
Robert Garrigus shot a 4-under 68 and Cameron Beckman carded a 1-under 71 to tie at minus-12. Darron Stiles (68) and Brian Bateman (69) share sixth at 11-under-par 277.
Any player in the top 30 -- including ties -- after Monday's final round will receive a PGA TOUR card for next season. The following 50 will earn full privileges on the Nationwide Tour.
Golfers rotate over the Stadium Course at PGA West and the Jack Nicklaus Tournament Course. There is no cut as all players play each course three times in this 108-hole marathon.
Boyd, who has only made six starts on the Nationwide Tour in his career, teed it up at the Stadium Course on Saturday and tallied his first birdie at the par-4 third hole. He closed his front nine in style with back-to-back birdies from the eighth to make the turn at 3 under par.
He started off strong on his second nine as well with two birdies in a row from the 11th. Boyd tallied his final birdie at the par-5 16th in his attempt to earn medalist honors Monday afternoon.
'I'm trying to keep my head down and keep playing,' said Boyd, who said he told his wife earlier in the year that he would get a 'real' job if he did not get a place to play this year.
Barcelo also played the PGA West Stadium Course in round four and his round began with six consecutive pars. He birdied both the seventh and eighth holes to reach 13 under par for the championship.
He birdied the 12th hole to join Boyd in the lead, then coasted with pars. At the last, Barcelo left himself with a little tester, but converted the 3-footer for par and a share of the lead.
'This was the game plan - play smart and play safe,' said Barcelo, who fired a 64 on the Stadium Course on Thursday. 'My play this week has just been fantastic.'
Overnight leader Paul Gow struggled on Saturday. He shot a 3-over 75 and is tied for eighth place with John Merrick (67) and Bob Heintz (71) at minus-10.
Saturday's biggest mover was Stephen Marino. At the Jack Nicklaus Course, he recorded 11 birdies and a bogey for a 10-under 62. That low score moved him from a tie for 80th, to a tie for 19th at 7-under-par 281.