Put on plastic gloves to protect your hands and rinse the plastic mat with hot water to remove loose stains.
Wet the scrubbing sponge with hot water and sprinkle baking soda onto the plastic mat until it is completely covered.
Scrub the rough side of the sponge against the sink mat. Leave the mat in the sink while cleaning to avoid spilling water and baking soda onto the counter or floors.
Turn on the kitchen sink's hot water and rinse the plastic mat under the running water.
Blot the plastic mat dry with a dry, clean cloth. If the stain is gone no further action is required, but if the tomato stain remains the mat must be bleached.
Insert the sink stopper into the sink's drain and fill the sink with enough hot water to cover the mat.
Add 1/2 cup of bleach to the sink and leave the mat in the sink overnight.
Remove the sink stopper to drain out the cleaning solution and rinse the plastic mat thoroughly under hot, running water.
Wipe the mat dry with a cleaning cloth to remove remaining moisture.