Homicides rose in most big American cities in 2016, continuing a worrisome trend for police and criminologists that began last year, even as murder rates in most cities are nowhere near the levels of two decades ago.

Sixteen of the 20 largest police departments reported a year-over-year rise in homicides as of mid-December, a Wall Street Journal survey found. Some notched minor increases, while Chicago saw one of the most dramatic jumps, with more than 720 murders—up 56% from 2015.

Nationally,...