The 59-year-old will come in as a replacement for current boss Louis Van Gaal, who announced earlier this week that he has been undergoing treatment for prostate cancer.
Koeman was previously in charge of Netherlands between 2018 and 2020.
He guided Netherlands to qualification for EURO 2020 but after the tournament was delayed by a year because of the coronavirus pandemic, Koeman stepped down to take charge of Barcelona.
Koeman has been out of work since being sacked by the Blaugrana back in October.
Koeman said after the announcement was made on Wednesday: “I am looking forward to the new collaboration. A little over a year and a half ago, I certainly did not leave the Dutch national team out of dissatisfaction.
"My stay felt good, the results were good and the click with the internationals was good. We will soon continue on that path. That is certain for me.”
The KNVB's director of professional football, Marianne van Leeuwen, added: "We are very happy that Ronald will return next year. During his previous 'term' as national coach, there was great satisfaction with his work and the results.
"That is why, after internal deliberations, I entered into discussions with Ronald and his agent Rob Jansen, together with director of top football Nico-Jan Hoogma.”
“I did not want to wait to fill in the succession of Louis van Gaal," he added.
"After all, the changing of the guard will come at an atypical moment; end of the calendar year, a period in which top trainers have been under contract for a long time.
"As early as mid-January, we therefore decided to contact Ronald, who was available. Then we came to an agreement in a few conversations."
Netherlands are in the same group with host nation Qatar, Africa champions Senegal and Ecuador at the World Cup.
