The rainbow smelt prefer juvenile ciscoes, zooplankton such as calanoid copepods (Leptodiaptomus ashlandi, L. minutus, L. sicilis), and other small organisms, but are aggressive and will eat almost any fish they find.

They are weak swimmers and struggle to navigate fish ladders preventing them from making it past dams to the headwater streams where they spawn.

It has a silvery, pale green back and is iridescent purple, blue, and pink on the sides, with a light underside.

The rainbow smelt has been introduced into water bodies in the U.S. states of Colorado, Connecticut, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Montana, Nebraska, New Hampshire, New York, North Carolina, North Dakota, Ohio, Pennsylvania, South Dakota, Tennessee, Vermont, Virginia, and Wisconsin.

In 1883 Stedman and Argyle found that the rainbow smelt consumed bloaters (Coregonus hoyi) and alewives (Alosa pseudoharengus).

[5] A 2003 study by Horppila et al. shows that smelt densities can exceed 40,000 individuals per hectare and may create a large predation pressure on the lake.

[6] Another study showed that in Lake Ontario the primary food sources for rainbow smelt were slimy sculpins (Cottus cognatus) and opossum shrimp (Mysis relicta), but nothing was said on whether this predation had significant impact on the populations.

It was also shown by Brandt and Madon in 1986 that adult lake trout (Salvalinus namaycush) could be a keystone predator species for the smelt.

[5] The U.S. EPA stated in 2008 that they believe the smelt contributed to the extinction of the blue pike (Stizostedion vitreum glaucum) by outcompeting for food.

Landlocked adult rainbow smelt spawn shortly after ice-off at night in the lower reaches of streams.

[6] Crossman and Scott state that in optimal conditions and large lakes, rainbow smelt may reach 35.6 centimetres (14.0 in) and can live for over seven years.

[9] The populations of the rainbow smelt in areas where it has been introduced, such as the Great Lakes, have been increasing in many regions, even with efforts to control its spread.