The investigation by CNN reported that the ads ran on YouTube channels that promoted white nationalists, Nazis, paedophilia and North Korean propaganda.
Besides Under Armour, other brands named include Adidas, Amazon, Cisco, Facebook, Hilton, LinkedIn, Netflix, and Hershey.
Ads from several departments of the US government were also affected.
CNN reported that the brands it contacted were unaware that they had ads running on these YouTube channel.
When contacted for comment, a Google spokesman said Google was aware that "even when videos meet our advertiser-friendly guidelines, not all videos will be appropriate for all brands. But we are committed to working with our advertisers and getting this right."
"We have partnered with our advertisers to make significant changes to how we approach monetisation on YouTube with stricter policies, better controls and greater transparency. When we find that ads mistakenly ran against content that doesn't comply with our policies, we immediately remove those ads," the spokesman said.