Southwest

The four main terminals at the SFO each cater to specific passengers. The first three terminals, labeled from one to three, cater to fliers of domestic flights. They also cater to passengers who are flying to and from Canada.

The last terminal, as expected, is the International Terminal. This terminal obviously caters to passengers of international flights.

Arrivals Terminal:

With the knowledge of the domestic versus the international terminals, it makes sense that Southwest Airlines, which is a Texas-based company, would operate from Terminal 1.

Thus, if the SFO is your destination of choice via Southwest, then you are most likely to be greeted at Terminal 1. However, this may be subject to change, which is why it is always best to check ahead of time.

To check the details of your Southwest flight arriving at the SFO, click here.

Departures Terminal:

Similarly, your Southwest flights leaving from the SFO will also most likely operate from Terminal 1. Again, this may not always be the case, so do check on your flight information prior to your arrival at the airport. This may save you time in transferring between terminals, and you may even be able to park more conveniently at your designated terminal.
At Terminal 1, find your Southwest gate at the second level.

To find out which SFO terminal your Southwest flight will fly from, click here.

Flights

To find out all the information you need about SFO flights and their schedules at the airport, click here.

Codeshare

Southwest Airlines does not codeshare with any other airline companies at the SFO. This is the reason as to why your flight is less likely to be found at a terminal other than Terminal 1.
However, there may be other reasons for the change of terminals that are not accounted for. So, do click here to find out the status of your flight and the SFO terminal that it will land at or depart from.