The first fire occurred in a mailbox standing near the corner of Sansome and Bush streets at around 2 a.m., police said.
The second mailbox burned on the 900 block of Pine Street at around 2:40 a.m., police said.
Arson investigators were called to both scenes as it appears they were deliberately set, police said.
The fires were contained to the inside of the mailboxes, and no other damage was caused.
No one was injured.