Zack Willett opened the scoring with his 35th league and cup goal of the campaign.
Midfielder Tommy Tierney wrapped up the win by scoring twice, with all the goals coming in the first half.
Horndean still have two games in hand on Portchester and travel to the On-Site Group Stadium next month.
Pictures by Keith Woodland
9. US Portsmouth (maroon/blue) v Horndean
US Portsmouth (maroon/blue) v Horndean. Picture: Keith Woodland Photo: Keith Woodland
10. Horndean's Zack Willett, left. Picture: Keith Woodland
Horndean's Zack Willett, left. Picture: Keith Woodland Photo: Keith Woodland
11. Zac Willett goes for goal
Zac Willett goes for goal. Picture: Keith Woodland Photo: Keith Woodland
12. US Portsmouth goalkeeper Owen Craig
US Portsmouth goalkeeper Owen Craig. Picture: Keith Woodland Photo: Keith Woodland
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