The Hornets, of the Isthmian premier division, were too strong for their SCFL premier opponents, who were not helped by a controversial first half red card.
Horsham will play Littlehampton or Hastings, who meet in the second semi, also at Lancing, on March 12.
See pictures by John Lines and Natalie Mayhew (Butterfly Football) on this page and those linked.
1. Horsham v Steyning Town pictures by Natalie Mayhew-Butterfly Football (7).jpg
Action from Horsham v Steyning Town in the Sussex Senior Cup semi-final Photo: Natalie Mayhew-Butterfly Football
2. Horsham v Steyning Town pictures by Natalie Mayhew-Butterfly Football (37).jpg
Action from Horsham v Steyning Town in the Sussex Senior Cup semi-final Photo: Natalie Mayhew-Butterfly Football
3. Horsham v Steyning Town pictures by Natalie Mayhew-Butterfly Football (11).jpg
Action from Horsham v Steyning Town in the Sussex Senior Cup semi-final Photo: Natalie Mayhew-Butterfly Football
4. Horsham v Steyning Town pictures by Natalie Mayhew-Butterfly Football (5).jpg
Action from Horsham v Steyning Town in the Sussex Senior Cup semi-final Photo: Natalie Mayhew-Butterfly Football