Prepare mason jars and lids by washing and sterilizing in boiling water or running through the sterilization cycle on the dishwasher.
Wash strawberries and rhubarb. Slice strawberries into halves or quarters and then crush them with a potato masher. Measure out 3 cups precisely and put into a large saucepan.
Cut rhubarb into small pieces. You do not need to peel the rhubarb in advance. Place the rhubarb into the saucepan also.
Add sugar and pectin and begin heating over medium high heat.
Cook, stirring frequently, until mixture is broken down, juicy, and bubbling. Bring jam to a full boil for 4 minutes and then remove from heat.
Ladle jam into the prepared jelly jars, secure the lids, and leave the jars to set for 24 hours before storing in the freezer.
Alternately, bring a tall stockpot of water to a full rolling boil. Submerge jam jars under the boiling water bath for 15 minutes.
Remove from the boiling water and set on the countertop for 24 hours.
After 24 hours, tighten the lids completely and store processed jam on the shelf for up to 1 year. Refrigerate after opening for up to three weeks.