Howley was promoted with immediate effect after it was confirmed New Zelaander Gatland would relinquish his post for a year to lead the British and Irish Lions in New Zealand.
It is a similar scenario to 2013 when Gatland took the Lions to Australia and Howley led the Welsh to the Six Nations title, although this time the former Wales captain will be in charge for November's tests as well as next year's tournament.
"We are delighted to appoint Rob as Head Coach of Wales for the period that Warren is away," Phillips said.
"We considered a range of options domestically and overseas when contingency planning for this eventuality and determined that Rob represents the optimum appointment for Wales.
"Rob's experience and relationship with the regional coaches affords the opportunity for a joined up approach."
Howley has been part of Wales' coaching set-up since 2008.
(Reporting by Martyn Herman; editing by Ken Ferris)
