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.
37. Horsham v Steyning Town in the Sussex Senior Cup semi-final Pictures by John Lines (32).JPG
Horsham take on and beat Steyning in the semi-final of the Sussex Senior Cup Photo: John Lines
38. Horsham v Steyning Town in the Sussex Senior Cup semi-final Pictures by John Lines (88).JPG
Horsham take on and beat Steyning in the semi-final of the Sussex Senior Cup Photo: John Lines
39. Horsham v Steyning Town in the Sussex Senior Cup semi-final Pictures by John Lines (94).JPG
Horsham take on and beat Steyning in the semi-final of the Sussex Senior Cup Photo: John Lines
40. Horsham v Steyning Town in the Sussex Senior Cup semi-final Pictures by John Lines (92).JPG
Horsham take on and beat Steyning in the semi-final of the Sussex Senior Cup Photo: John Lines
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