One flight specifically mentioned was Flight 4200 from San Diego to Dallas Love Field.
The threat came from a Twitter account claiming to be an ISIS support group.
It warned there was a semi-nuclear explosive on board the plane, and if it failed, the plane would be shot down.
Flight 4200 landed safely at Love Field without incident around 8 p.m. Monday.
Southwest responded to that tweet and a few other threats with the same response, saying, "Any threat or alleged threat against the safety of our employees, customers, or aircraft is a matter we take very seriously. We are working with the relevant authorities to ensure the safety of this and every flight."
Monday's threat marks the third straight day of social media threats against domestic airlines.
So far, they've only been threats.