The former Springbok star joined the Ospreys on loan from Super 14 side The Stormers in November and played in four group matches as Scott Johnson's men beat Viadana in back-to-back ties before they lost to Clermont Auvergne and then toppled Leicester.
ERC regulations state that a player 'must have at least a three-month contract with the club' in order to be eligible for European competition.
Januarie, however, returned to South Africa in January, less than three months after arriving in Wales.
Premier Rugby, the umbrella body for Guinness Premiership clubs in England, reportedly issued a complaint and ERC ordered an investigation.
That investigation concluded on Monday in Dublin with the Ospreys told they had 'no case to answer'.
"Having co-operated fully with ERC at all times during their investigation into this matter, and providing them with all the relevant paperwork relating to Ricky's contract and registration, we are delighted to have been informed that they have found that Ospreys Rugby has no case to answer," said Ospreys managing director Roger Blyth.
"Now that we can finally put this matter behind us, we are, along with our many supporters, looking forward to our quarter-final against Biarritz in San Sebastian on Saturday 10th April."