The mourning ended on Monday after a five-day funeral last week, and many citizens have returned to wearing coloured garments after having stuck to black for a year.
Participants in the festival pay respects to the water spirits, floating small, traditionally-hand crafted rafts carrying flowers, bamboo and candles on waterways in an ancient ritual.
The rafts are also thought to carry away bad luck and usher in good fortune during the festival, which is celebrated all over Thailand, but is particularly spectacular in the north.
Festivities will stay in a low key this year, out of respect for the late monarch, however, authorities in the capital said.
“We will not have excessive celebrations but will focus more on showcasing Thai culture,” Chalermpon Chotinuchit, of the Bangkok Metropolitan Administration (BMA), told Reuters.
The BMA urged celebrants to use rafts made from natural materials this year, after environment groups flagged concerns about non-biodegradable components clogging rivers and drains.
Festival participants also struck a sombre note.
“It will be a sadder period this Loy Krathong because everyone loves the late King Rama IX,” said 23-year-old Nanthicha Sorndee.
Safety concerns have prompted national carrier Thai Airways International to cancel some roundtrip flights between Bangkok and Chiang Mai on Friday and Saturday, it said in a statement, without elaborating.
The northern Thai city celebrates Loy Krathong alongside Yi Peng, a festival in which brightly lit lanterns are wafted into the sky.