Mrs Seymour, who is the party's transport spokesman, is supporting Henry Bolton in the party's leadership election on September 29.
It will be the party's third leadership contest in just over a year.
Mrs Seymour's fellow MEP for the West Midlands, Bill Etheridge, withdrew from the race saying there were too many candidates.
Mr Bolton is one of 10 candidates seeking to lead the Eurosceptic party.
Mrs Seymour, who is based in Wellington, said Mr Bolton was the most credible candidate to lead the party into a new era.
“Having been involved with Ukip for more than 17 years, I’m in no doubt that Henry has the experience, respect and skill-set required to reform and relaunch our party," she said.
"He is diligent, politically astute, calm and steady under fire, and with his police and military background, his organisational and strategic planning skills are second to none.
“Henry isn’t swayed, tainted or influenced by political ideology, and knows how to make pragmatic, sensible and reasoned decisions."
She said Mr Bolton also had vast international experience having held top-level diplomatic posts across in Afghanistan, Libya, Kosovo, Bosnia.
He was appointed an OBE for services to international security and stabilisation.
"He is a proven problem solver and collaborator," Mrs Seymour added.
Mr Bolton has also been endorsed by former Ukip leader Nigel Farage.
Mrs Seymour said: “Critics accuse Ukip of being insular, and inward looking. Henry Bolton is the exact opposite.
“Grassroots members and loyal donors are the beating heart of our party, and Henry will listen to them, and collaborate with them, to chart UKIP’s future course."
Ukip elected Diane James as its leader on September 16 last year, but she resigned after just 18 days.
Mr Farage briefly returned as acting leader, before Paul Nutall won a second leadership election last year on November 28.
The present election is being held following Mr Nutall's resignation on June 9, following a disappointing showing in the General Election.
Mr Etheridge is standing to be deputy leader of the party, as running mate to John Rees-Evans, who is also a former soldier.