Plebejus idas, the Idas blue or northern blue, is a butterfly of the family Lycaenidae.

This species can be found in most of Europe (except parts of Spain, southern Italy and the United Kingdom), in the northern regions of the Palearctic (Siberia, mountains of South Siberia and Yakutia) and in the Nearctic realms.

Plebejus idas has a wingspan of 17–28 mm. This species is quite variable in colors and markings. The upperside of male's wings is iridescent blue, while it is brown with orange submarginal spots in the females. The underface of the wings is greyish with black spots and it shows a thin black line and small dots along outer margin. Along these margins are also present a rather large orange band with blue spots. This species is very similar to the Silver studded blue (Plebejus argus) and to the Reverdin's blue (Plebejus argyrognomon). The forelegs of male of Plebejus idas lacks a hook which is present in Plebejus argus.

The species flies in a single brood from June to August depending on location. The larvae feed on Calluna vulgaris, Vaccinium uliginosum, Empetrum nigrum and various Fabaceae species (mainly Cercis siliquastrum, Melilotus albus, Lotus corniculatus, Cytisus, Genista tinctoria, Trifolium pratense, Chrysaspis campestris, Astragalus alpinus and Anthyllis). They are usually attended by ants (Lasius and Formica species). Second-stage of the caterpillars overwinter.

A character of Margaret Atwood's MaddAddam book trilogy is named after the lotis blue butterfly.