Woods won five tournaments on the tour in a year when no other player won more than twice.
The wins -- the Farmer's Insurance Open in January, the Cadillac Championship and Arnold Palmer Invitational in March, the Player's Championship in May and the Bridgestone Invitational in August -- pushed Woods' number of PGA championships to 79. Only Sam Snead, with 82, has more.
The Player of the Year honor is officially called the Jack Nicklaus Award. Woods also won the Arnold Palmer Award as the tour's leading money winner with $8.5 million in 16 tournaments and the Vardon Trophy adjusted scoring average (68.985).
"It's been just a fantastic year all around," Woods said. "It's also an incredible feeling to be voted by your peers and have that type of respect. It's something that is very humbling."
Woods first won the PGA Player of the Year award in 1997 and took it nine of 11 years from 1999-2009. Nick Price, who won in 1993 and '94, is the only other player to win the award more than once since it began in 1990.
Spieth, 20, started 2013 with no status on any tour, but won the John Deere Classic in July. He is the youngest player to win a PGA Tour event since Ralph Guldahl in 1931. He tied Brandt Snedeker and Bill Hass for most Top 10 finishes this year (nine).
Others considering for PGA Rookie of the Year were Derek Ernst, Russell Henley and David Lingmerth.