from the OU website:
Rabbinical authorities disagree as to whether or not quinoa, the seeds of the goosefoot plant, is kitniyos. Those who permit it do so because it was not known to Jews when it became accepted to refrain from kitniyos. Others disagree and argue that it should be prohibited because it resembles other forms of kitniyos. Even if you follow the lenient opinion, we recommend sifting through your quinoa before using it to ensure that there are no chametz grains mixed in.