On the underside of the cap are small angular pores, initially bright yellow before turning greenish-brown with age.

A characteristic feature that helps to distinguish it from similar Suillus species, such as S. granulatus, is the pinkish mycelia at the base of the stem.

[6] A 1996 molecular analysis of 38 different Suillus species used the sequences of their internal transcribed spacers to infer phylogenetic relationships and clarify the taxonomy of the genus.

[1] In 2006, a phylogenetic analysis of Suillus isolates collected from Spain showed that S. collinitus was closely related to other species "typical of the Mediterranean area", namely S. bellinii, S. luteus, and S. mediterraneensis.

[8] The British botanist Mordecai Cubitt Cooke called the mushroom the "ringless yellow boletus" in an 1873 publication.

[10] The cap is covered with a brown cuticle of variable shade that is decorated with minuscule radial striations, which become more obvious in fully expanded specimens and especially in dry weather.

Young, fresh specimens sometimes bear droplets of a clear fluid that collect on the pore surface.

velatipes, originally found in Italy growing in poor acidic soil in association with Aleppo pine (Pinus halepensis), is distinguished by a glutinous brown veil at the base of the stem, and a slender white felting at the cap margin.

[6] Suillus collinitus is often found in the same habitat with other thermophilic species, including S. mediterraneensis and S. bellinii, especially in the Mediterranean basin.

It is an ectomycorrhizal fungus, forming mutually beneficial symbiotic relationships with several species of pine (Pinus).

Suillus collinitus is the most frequent fungus associating with Aleppo pine – a hardy tree utilised for reforestation schemes and soil conservation against erosion in the Mediterranean region.

A study performed in the south of France discovered that S. collinitus and S. mediterraneensis are multi-stage ectomycorrhizal fungi, associating with both young and mature stands of Aleppo pine.

Moreover, the fungus retained its viability to propagate via its mycelial network which helped the recolonisation effort of newly introduced trees.

[22][23] Unlike other Suillus species that grow in wet climates, such as S. grevillei[24] and S. spraguei,[25] S. collinitus does not typically produce fruit bodies in young plantations.

This fruiting behaviour may be attributed to the typically nutrient-poor xeric Mediterranean environment in combination with a lack of tree canopy.