In a large pot, combine the mashed peaches, sugar, and lemon juice. Stir well to combine.
Bring the mixture to a boil over medium-high heat, stirring frequently to prevent burning.
Once boiling, reduce the heat to medium and let it simmer for about 25-30 minutes or until the mixture has thickened to your desired consistency, stirring occasionally.
If using pectin, stir it in during the last 5 minutes of cooking.
Remove from heat and carefully pour the hot jam into sterilized jars, leaving about 1/4 inch of headspace. Seal the jars with lids.
Let the jars cool to room temperature before storing them in the refrigerator or processing them in a water bath for longer storage.