In September, Headley hit .324 with a .410 on-base percentage and had nine home runs and 30 RBIs. It follows an August in which he hit .306 with 10 home runs and had 31 RBIs.
"There's been so many more opportunities," Headley said of getting chances to hit with runners on base, the result of an improved overall offense. "The second half has been incredibly enjoyable, especially being around these guys."
Headley is the first player in franchise to knock in 30 or more runs in a month twice.
"It's a great honor, again. How often do you get to say that?" said Padres manager Bud Black. "Chase just kept going. Teams kept pitching him tougher, and he kept getting his hits."
Ken Caminiti is the only other player in franchise history to be named the NL Player of the Month twice in the same season (August-September, 1996).
Headley got a ringing endorsement from Brewers manager Ron Roenicke before Monday's game at Miller Park.
"To have that [production] in San Diego, that's pretty amazing," Roenicke said. "He needs to be talked about a lot more than he has been. To have that kind of year in that place? Phenomenal."
Going into Tuesday's game against the Brewers, Headley is hitting .283 with a career-best 31 home runs and 113 RBIs (also a career best) in 159 games. He leads the NL in RBIs, one ahead of Milwaukee's Ryan Braun.