The new rules take effect in about a month.
An "emotional support animal" will be limited to a single cat or dog, and must be kept in a carrier or on a leash.
Passengers must bring medical proof that the animal is necessary.
As for service animals, Southwest says it will only accept the most common ones - which are dogs, cats, and miniature horses.
Exotic animals will not be allowed to travel.
Southwest will also allow what are called "psychiatric support animals" as service animals.
In the past few months, United, Delta, American and Jet Blue all revised their animal policies.
Earlier this year passengers made headlines for being denied emotional support animals like hamsters and even a peacock on board planes.