No. That claim was wrong for decades. A careful 2016 recount found about 38 trillion bacteria and about 30 trillion human cells in a reference adult — roughly a 1-to-1 split, not 10 to 1.
In 1972, a microbiologist estimated the bacteria count by taking the density of bacteria in a gram of gut contents and scaling it up to the whole digestive tract. It was a rough order-of-magnitude guess — reasonable for its time, never meant to be exact. But it was catchy, so textbooks, articles, and TED talks repeated it for over 40 years until "you're 90% bacteria" sounded like settled science.
In 2016, researchers Ron Sender, Shai Fuchs, and Ron Milo went back and counted properly, organ by organ. Their bacteria total — about 38 trillion — came out close to the old guess. The human side is where the myth broke: the old math had badly underestimated your own cells. Most of them are red blood cells, and once those were counted honestly, the human total landed around 30 trillion.
One good sneeze's worth of rounding aside, you're about half you and half bacteria by cell count.
It changes the story, not your health. Your bacteria still matter enormously — they help digest food, train your immune system, and make vitamins — but they matter because of what they do, not because they outnumber you. By weight, by the way, it was never close: your bacteria together weigh only about 200 grams, roughly as much as a can of soup.
See the short version in the LOE PRESSURE "Things You Didn't Know" series — "You're Not 90% Bacteria — That Was Wrong for Decades."
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Sources: Sender R, Fuchs S, Milo R (2016), "Revised Estimates for the Number of Human and Bacteria Cells in the Body," PLoS Biology — https://doi.org/10.1371/journal.pbio.1002533; Phys.org — https://phys.org/news/2016-01-germs-humans-microbiome.amp. Figures are for a ~70 kg reference person and are approximate, matching the paper.