News of the 30 Rock star's engagement comes six months after he finalized his divorce from Monte Lapka, his partner of 13 years.
"They've already begun planning the wedding!" and insider told Us Weekly.
Jackson, 38, and Landau went public with their relationship in October 2013. The couple later attended the 2013 amfAR Inspiration Gala together in December.
This will be Jackson's second marriage, as he and Lapka tied the knot in New York in September 2011 after an 11-year relationship. The couple announced they had "mutually decided to divorce" in August of last year.