In a large kettle, combine sugar, plums, water and lemon juice.
Bring to a full rolling boil, stirring constantly.
Stir in pectin; return to a full rolling boil.
Boil 1 minute, stirring constantly.
Remove from the heat; stir in cinnamon.
Skim off foam.
Pour hot jam into hot jars, leaving 1/4-in.headspace.
Process for 10 minutes in a boiling-water bath.