"Today at 2:20 pm, our nation suffered a tragic air accident... I am deeply saddened to announce the passing of General Francis Omondi Ogolla, the Chief of the Kenya Defence Forces (CDF)," Ruto told reporters.
The president, who had convened an urgent meeting of the National Security Council after news of the accident emerged, said nine other "gallant military personnel" on board were also killed while two survived.
Ogolla, 61, a trained fighter pilot, had only been in the post for a year but was shortly due to mark 40 years of military service.
Ruto said the Kenya Air Force had dispatched an investigation team to establish the cause of the crash, which took place in Elgeyo Marakwet county, about 400 kilometres (250 miles) northwest of the capital Nairobi.
Ogolla's helicopter went down shortly after takeoff from the village of Chesegon, where he and his entourage had been visiting a school after making stops in other areas to see Kenyan troops and other sites.
Ogolla, a married father of two, was appointed Chief of the Defence Forces by Ruto in April last year, a position that also serves as the top military adviser to the president.
Under Kenyan military regulations, the defence chief normally retires at 62 or after four years in the post, whichever comes first.
Ruto told journalists last May that he appointed Ogolla despite him being among those who tried to overturn his narrow election win against opposition leader Raila Odinga in 2022.
"When I looked at his CV, he was the best person to be (a) general," Ruto said.
Ogolla joined the KDF in April 1984, rising through the ranks to command the Kenyan Air Force in 2018, a post he held for three years before becoming vice chief of the defence forces in 2021.
Ruto's predecessor Uhuru Kenyatta said Ogolla's death was a "significant loss to the nation".
"The CDF was not only an accomplished military leader, but also a devoted patriot who dedicated his life to serving and protecting our beloved country."