Rinse muscadine grapes and remove stems, then place in a large saucepan with water.
Bring grapes to a gentle boil over medium heat, stirring occasionally, until skins split and fruit softens (about 10 minutes).
Mash grapes slightly with a wooden spoon to release juice and flavor.
Strain the mixture through a jelly bag or fine mesh strainer to separate juice from pulp.
Return grape juice to the saucepan and stir in lemon juice and pectin until fully dissolved.
Bring to a rolling boil over medium-high heat, then add sugar gradually while stirring constantly.
Continue boiling hard for 1–2 minutes, ensuring sugar is fully dissolved and mixture thickens slightly.
Remove from heat and skim any foam from the surface.
Pour hot jelly into sterilized jars, leaving 1/4 inch headspace, and seal with lids.
Let jars cool at room temperature until set and fully chilled (about 6–8 hours) before storing.
