You can, but it is not ideal. Towels shed lint onto denim, and jean zippers and rivets can snag terrycloth loops.
Laundry day math tempts everyone to cram a half load of jeans in with bath towels to save water and time.
The catch is not the wash cycle itself, it is the collision between two very different fabrics: one sheds fibers, the other has hardware that grabs them.
Cotton towels release loose fibers every wash, more in the first 5 to 10 cycles than later on. Those fibers cling to zippers, rivets, and belt loops on jeans, and dark denim shows them as a dusty gray film after just one shared load.
Jean hardware works the other direction too, snagging towel loops and thinning the terrycloth pile over time, which is part of why towels washed with jeans for months start feeling rougher and less absorbent.
Turn jeans inside out and close zippers and buttons so hardware cannot grab fabric. Wash on cold to warm, about 86 to 104°F (30 to 40°C), rather than hot, since heat makes fibers cling more aggressively to denim's rougher weave. Run an extra rinse afterward if you notice lint stuck to either item, and shake both out before loading the dryer to knock loose fibers free first.
One load with jeans and towels together will not ruin anything. The real damage builds up over months of always washing them together: towels end up threadbare and noticeably less absorbent within a year or two of regular use, and jeans pick up permanent lint that even a lint roller cannot fully remove, plus faster fading from the extra friction of tumbling against heavy terrycloth.
Other sturdy dark items work best alongside jeans: dark t-shirts, hoodies, and other cotton pants hold up to the same water temperature and cycle length without the lint problem towels bring. If you only have a small load of jeans, adding one or two similar-weight dark garments is usually a better fit than reaching for towels, and it still lets you run a full, efficient load instead of washing jeans alone.
Jeans and towels can share a wash occasionally without harm, but the two fabrics genuinely clean better apart. If your machine handles small loads well, keep a separate pile for denim.
Not from one wash, but repeated cycles with zippers and rivets can snag and thin terrycloth loops over months, shortening a towel's life.
Skip it for the first two or three washes. New dark jeans release excess dye that can tint light-colored towels, especially white ones.
Use a normal or gentle cycle in cold to warm water, 86 to 104°F, with jeans turned inside out and zippers closed to reduce snagging.
It is better to dry them separately too. Towels dry faster, and jeans can end up over-dried or shrink if left in for a towel-length cycle.