The change took effect late last week.
"On flights where adjacent seats are available, our agents at the airport are empowered to provide an additional seat at no extra cost to Customers who require one," the airline said in a statement. "If another seat is not available, we will work to accommodate the Customer on a later flight."
Earlier this year, Southwest introduced a policy requiring passengers who needed additional space to purchase a second seat before traveling. Customers could later request a refund if extra seats were available on the flight.
Under the new policy, passengers who do not book an extra seat in advance still risk not being able to board their original flight if no additional seats are available.
When the earlier extra-seat policy took effect in January, Southwest faced criticism from some travelers who said they were told at ticket counters or gate areas that they would not fit in a single seat.
The updated policy is expected to reduce the need for airport employees to determine whether a passenger may require additional seating at the gate.
Southwest said it still encourages customers who may need an extra seat to book one in advance to help avoid last-minute issues at the airport.
The policy adjustment followed an extensive review and that it will continue refining the process "to create a more consistent and seamless travel experience for customers who require an additional seat," the airline said in a statement.
"We continue to encourage Customers who need an additional seat to book it in advance to help alleviate any last-minute inconvenience at the airport," the statement added.