There are 296 seven-letter words that start with J in the ENABLE word list used by this site. That is about 1.3% of all seven-letter words. The highest-scoring is jazzily, worth 35 points in standard Scrabble tile values.
Different games use different word lists. Check unusual words against the list your game uses.
Highest-scoring seven-letter words starting with J
jazzily35 points
jazzman34 points
jazzmen34 points
jazzing33 points
jazzers32 points
jazzier32 points
jukebox27 points
jezebel25 points
jimjams25 points
jackdaw24 points
jaywalk24 points
jumbuck24 points
jumpoff24 points
jejunum23 points
jewfish23 points
jezails23 points
jockeys23 points
jojobas23 points
jonquil23 points
jujubes23 points
jujuism23 points
jackpot22 points
jinxing22 points
jubhahs22 points
Patterns in this list
The most common second letters are ja (80), ju (70), jo (58), je (54), ji (34). 140 of these words contain three or more vowels, which can make them easier to spot when you are unscrambling.