"We have loved each other for a very long time and we share four incredible sons together," Gilbert said in a statement.
The couple have a 15-year-old son and they're seeking joint custody of him. Gilbert has a son from a previous marriage and Boxleitner has two sons from his marriage to Kathryn Oglivy.
On TV, the couple co-starred in three episodes of 'Babylon 5' in 1996.
Aside from 'Little House,' Gilbert has made appearances on '7th Heaven,' 'Nip/Tuck' and voiced Batgirl in 'Batman: The Animated Series.' Boxleitner recently popped up on 'Chuck' and 'NCIS.'
'Little House' is still airing in reruns on Hallmark Channel.