In 1990, he married Brazilian-born Beatriz Karam. They have four children:
Lady Gabriella Helen Seymour (b. 1992)
William Francis Seymour, Earl of Yarmouth (b. 2 November 1993). He married Kelsey Wells and they have two sons: Clement Andrew, Viscount Beauchamp (b. 2019) and The Honourable Jocelyn William (b. 2021)
Lord Edward George Seymour (b. 1995)
Lady Antonia Louisa Seymour (b. 1998)
Arms
Coat of arms of Henry Seymour, 9th Marquess of Hertford
Crest
1st: the Bust of a Moor in profile couped at the shoulders proper and wreathed about the temples Argent and Azure (Conway); 2nd: out of a Ducal Coronet Or a Phoenix of the last issuing from flames proper (Seymour)
Escutcheon
Quarterly: 1st and 4th, Sable on a Bend cotised Argent a Rose Gules between two Annulets of the first (Conway); 2nd and 3rd, quarterly: 1st and 4th, Or on a Pile Gules between six Fleurs-de-lis Azure three Lions of England (being the Coat of Augmentation granted by King Henry VIII on his marriage with Lady Jane Seymour); 2nd and 3rd, Gules two Wings joined in lure the tips downwards Or (Seymour)
Supporters
On either side a Blackamoor wreathed about the temples Or and Sable habited in short golden garments and in buskins gold adorned about their waists with green and red feathers each holding in his exterior hand a Shield Azure garnished Or the dexter charged with a Sun in Splendour gold and the sinister with a Crescent Argent