The 21-year-old started three out of the four games during the busy month, which also included the home PRO12 victory over the Newport Gwent Dragons.
With 45 caps (43 starts) in just two seasons, Marmion has not missed a game since his senior debut against Cardiff Blues in September 2012 and is among this season's top try scorers for Connacht.
The English-born scrum half, who was raised in Brecon, Wales, was recruited to Nigel Carolan's Connacht Academy in 2011 before integrating fully into the senior set-up.
He recently signed a contract extension that will see him play with Connacht until at least the summer of 2016.
A member of Ireland's extended squad since the 2013 summer tour to North America, Marmion has yet to win an international cap, but has been included in Joe Schmidt's 44-man training squad for the upcoming Six Nations and O2 Ireland Wolfhounds game against England Saxons.
Marmion has previously been capped at Under-18, Under-19 and Under-20 levels for Ireland.
Commenting on the province's Player of the Month for December, Connacht head coach Pat Lam said: "Kieran has been so consistent for us all season. He puts in the hard work and is constantly striving for improvement.
"He was once again a fantastic team player in December and it's difficult not to single him out in conversations around Player of the Month.
"His mental toughness was brought to the fore last month when, 48 hours after being hospitalised on a drip with the 'flu, he went on to play 80 minutes in our home Heineken Cup game against Toulouse when we had no half-backs left standing.
"He deserves this accolade and he deserves his call-up to the Ireland squad this week. I have no doubt he will go on to achieve great things in rugby with both Connacht and Ireland."
Richard Hartmann, Director of Hartmann Jewellers, added: "We're getting to know Kieran quite well here in Hartmann's as this is his third Player of the Month award. It's always great to have him into the store and we hope his mum enjoys the watch he picked out for her!
"December was such a wonderful month for Connacht supporters with the famous win over in Toulouse. We're still talking about that game and Kieran's super try.
"He's a fantastic ambassador for the west of Ireland and Connacht Rugby and hopefully he will be lining out for Ireland very soon."