The stabbing occurred just before 10 a.m. at the park, located in the city's Haight-Ashbury neighborhood near Baker and Haight streets, police spokesman Officer Albie Esparza said.
Esparza said the victim, who is believed to be between 30 and 40 years old, was drinking in the park with two other homeless men when an argument began and one of the men stabbed him in the chest.
The suspect, Michael Little, was later arrested, homicide Lt. Mike Stasko said.
The argument appears to have been over a woman, Stasko said.
Others carried the victim down to the street and called police, but the man died at the scene, Esparza said.
The medical examiner's office had not yet identified the man this morning.
Little, 30, was booked into jail on a murder charge and remains in custody, according to the sheriff's department.