The birth of the pup was a surprise to aquarium officials, who were not aware of the pregnancy.
Monterey Bay explained in a blog post that for the last several days, "a wild female sea otter had been using the protected basin of our Great Tide Pool to rest from the winter storms."
PHOTOS: Monterey Bay Aquarium welcomes new sea otter pup
The aquarium added that it's "rare for a healthy sea otter to visit the pool so frequently" that staff "started to wonder if she was doing all right."
The mystery was solved Sunday morning when aquarium staff witnessed a brand new pup resting on mom's belly "being furiously groomed by a proud momma," the blog stated.
The aquarium notes in its blog that "not that long ago, sea otters were hunted to near extinction. Maybe 50 were left in all of California by the early 1800's.
But now, thanks to legislative protection and a change of heart toward these furriest of sea creatures, the otter population has rebounded to steady levels in the Monterey Bay, and with 3,000 total in central California."
ABC News contributed to this report.