The openside flanker helped Cardiff reach third place in the Premiership with wins over Aberavon and Ebbw Vale last month.
The recipient for the Principality Premiership monthly award is chosen by Premiership coaches and WRU elite performance staff, and the winner receives £100 from Premiership title sponsors Principality Building Society.
Shellard puts his impressive performances of late down to Phil Davies' recent influence at the capital club. He said, "Since Phil Davies arrived at the club, he has given us the confidence to go out and play rugby - and the open style of play he advocates also suits me down to the ground.
"Phil has brought a professional edge to the set-up and we are now meeting for training three times a week in addition to our individual work.
"The upturn in our results on the field means we now have real ambitions to win the league. We know that if we can keep winning, we have a side that can achieve that goal for the first time since the Premiership began."
Shellard was in action for Wales at the Hong Kong Sevens this weekend and now travels to Adelaide for the next leg of the IRB Sevens World Series. He said, "I've no doubt that my performances in the Principality Premiership got me noticed and included in the Wales Sevens squad.
"There have been a lot of changes in the squad since the World Cup win, but there is a still a really good buzz in the camp. I've been training with the squad all season and it's great to get a chance on the circuit again. I've played for Wales Sevens at the Hong Kong tournament twice already and it's a fantastic tournament. As a new squad, we were pleased with a lot of our rugby in Hong Kong and now we're looking to build on that in Adelaide."