Take the cotton sweater to a sink. Hold the stained area under the faucet and rinse it with warm water.
Pour laundry detergent directly on the tomato sauce stain. Use enough to cover the stain completely. Gently rub the detergent into both sides of the stain.
Wash the cotton sweater in your washing machine along with other clothing items that are the same color. Use warm water and add detergent plus chlorine laundry bleach if the sweater is white. For a colored sweater, use detergent with color-safe bleach.
Remove the sweater from the machine and inspect it to make sure the stain has been removed. If you still notice some discoloration, rewash the sweater.
Hang the sweater to dry, dry it flat on a towel, or dry it in your clothes dryer.