Both players missed this season's entire 6 Nations Championship campaign due to injury, but they have been named in a 32-man training squad by tour head coach Robin McBryde.
"We have selected a squad with the right blend of experience and youth, and this is a great opportunity for players to make their mark on the international stage," he said.
"It's a chance for some players to experience the international environment for the first time, but it is also an opportunity for players who have been involved previously to take the next step as we look to develop leaders.
"We are travelling to Japan as 6 Nations Champions and are focussed on the two-Tests that await us there. We will be looking to finish the season on a high."
The group will be reduced to 27 players nearer departure. Wales play Japan in Osaka on June 8, and then in Tokyo seven days later. The training party features 11 uncapped players, with the announcement coming just 24 hours after British and Irish Lions head coach Warren Gatland selected 15 Welshmen in his squad for this summer's tour to Hong Kong and Australia.
Fly-half Dan Biggar, the only player who started Wales' recent record 30-3 victory over England not to gain Lions selection, joins Priestland in the group as experienced number 10 options for McBryde.
Long-serving internationals Ryan Jones, Matthew Rees and James Hook are all rested, but they will be available to join the Lions in the event of injuries, if required.
Backs: S Shingler (London Irish), T Prydie (Newport Gwent Dragons), H Robinson (Cardiff Blues), Liam Williams (Scarlets), D Howells (Ospreys), J Spratt (Ospreys), O Williams (Cardiff Blues), A Beck (Ospreys), S Williams (Scarlets), D Biggar (Ospreys), R Priestland (Scarlets), Lloyd Williams (Cardiff Blues), T Knoyle (Scarlets), A Davis (Scarlets).
Forwards: R Bevington (Ospreys), R Gill (Saracens), S Hobbs (Cardiff Blues), S Andrews (Cardiff Blues), C Mitchell (Exeter), R Jones (Scarlets), K Owens (Scarlets), E Phillips (Scarlets), S Baldwin (Ospreys), B Davies (Cardiff Blues), L Reed (Cardiff Blues), A Coombs (Newport Gwent Dragons), J King (Ospreys), A Shingler (Scarlets), D Baker (Ospreys), R McCusker (Scarlets), A Pretorius (Cardiff Blues), J Navidi (Cardiff Blues).