Thesium strictum inflorescences grow in branched, dense clusters at stem-tips. The white open flowers have five spreading lobes that curve at their tips like the pointed arch or ogive of a Gothic vault. The lobe margins are ciliate, the white hairs short. The perianth base is a five-angled green cup.

The five yellow anthers are chunky, positioned inside the lobe base centres and at about cup height. They are slightly taller than the greenish stigma in the centre, which is a little smaller than an anther (Vlok and Schutte-Vlok, 2015; Privett and Lutzeyer, 2010; iNaturalist).