Forsythe hit a 1-0 pitch from Steve Edlefsen (0-1) into the second deck in left field.
San Diego's Carlos Quentin hit his first two home runs at Petco Park, giving him five since coming off the disabled list last week.
Lincecum is winless in seven starts, including three no-decisions and four losses. The Giants had rallied from a 4-0 deficit to take a 5-4 lead in the seventh, when Lincecum was lifted for pinch-hitter Aubrey Huff. Brandon Crawford singled to center, stole second and scored the go-ahead run on Theriot's two-out single to right.
The Padres had runners on first and second with no outs in the seventh before former teammate Clay Hensley retired the side, including two strikeouts.
Theriot had three hits and three RBIs while Gregor Blanco had three hits.
Lincecum allowed four runs and five hits in six innings, struck out eight and walked one. He is 0-4 with three no-decisions and a 5.90 ERA in seven starts since April 28.
The Padres jumped on Lincecum for four runs in the second, including Quentin's leadoff homer to deep left. Chase Headley followed with a double and scored on Everth Cabrera's one-out single. Cameron Maybin had a two-out, broken-bat double with two outs.
That lead was gone by the sixth.
Blanco tripled leading off the Giants' third and scored on Theriot's single to left. The Giants then scored three runs in the sixth to tie it. Theriot doubled in Blanco, advanced when Melky Cabrera beat out a bunt for a single and scored on Buster Posey's sacrifice fly. Angel Pagan singled to chase starter Anthony Bass and Brandon Belt greeted hit a sacrifice fly off Andrew Casher.
Bass allowed four runs and seven hits in 5 1-3 innings, struck out four and walked one.
NOTES: Street was reinstated from the 15-day disabled list Tuesday and RHP Miles Mikolas was optioned to Triple-A Tucson. Street was placed on the DL May 5 with a strained right lat muscle. In two rehabilitation starts for Class-A Lake Elsinore, he posted a 9.00 ERA. Mikolas made his major league debut on May 5. In 13 relief outings he was 1-1 with a 3.29 ERA. ... The three-game series continues Wednesday with a matchup of lefties between San Francisco's Madison Bumgarner (6-4, 3.04) and San Diego's Clayton Richard (2-6, 4.44).
- San Francisco Giants
- San Diego Padres
- Tim Lincecum
- Buster Posey
- San Diego
- Carlos Quentin