Users who filed complaints argued that Electronic Arts and BioWare misled fans about the impact player choices had on the game's multiple endings.
In its ruling, the ASA said that the complaints would not be upheld, because the endings were "thematically quite different".
"The ASA acknowledged the complainants' belief that players' choices in the game did not influence the outcome to the extent claimed by EA," reads the ASA statement.
"However, we considered that the three choices at the end of the game were thematically quite different, and that the availability and effectiveness of those choices would be directly determined by a player's EMS score, which was calculated with reference to previous performance in the game(s).
"We also acknowledged that there appeared to be a large number of minor variations in the end stages of ME3, and that those were directly impacted by choices made by players earlier in the game(s).
"Whilst we acknowledged that the advertiser had placed particular emphasis on the role that player choices would play in determining the outcome of the game, we considered that most consumers would realise there would be a finite number of possible outcomes within the game and, because we considered that the advertiser had shown that players' previous choices and performance would impact on the ending of the game, we concluded that the ad was not misleading."
Despite the ruling, BioWare and EA will launch the free 'Extended Cut' DLC, which will expand the ending through additional cinematic sequences and epilogues.
The aim of the pack is to provide additional closure relating to how each player's personal journey ends. It will be released this summer.
North American Mass Effect 3 fans previously filed complaints with the US Federal Trade Commission over the game's controversial ending.
US watchdog the Better Business Bureau agreed with fans, saying that the game did not fulfil the promises set out by its advertising.
Mass Effect 3 shipped 3.5 million copies worldwide during its first week on sale, making it the biggest entertainment launch of the year, according to publisher Electronic Arts.
Mass Effect 3 is available on Xbox 360, PS3 and PC.
> Read our review of Mass Effect 3
Watch the launch trailer for Mass Effect 3 below: