The Siskin is mainly a granivore although it varies its diet depending on the season. It feeds in trees avoiding eating on the ground.

In autumn and winter its diet is based on the seeds of deciduous trees such as birch and above all alder. They also visit cultivated areas and pasture where they join with other finches in eating the seeds of various Compositae such as thistles, dandelions, artemisias, knapweeds and other herbaceous plants such as St John's wort, meadowsweet and sorrel.

In spring, during the breeding season, they are found in coniferous forests. At this time their feeding is based on the seeds of these trees, especially on trees belonging to the genera Abies, Picea and Larix.They also feed on elms and poplars. When feeding the young they eat more insects, mainly beetles, as the proteins they contain help the chicks to grow. In summer their feeding is more varied, adding other herbaceous plants to their diet of conifer seeds: goosefoots and other Compositae.