The nucleotide sequences (18S and rbcL) of the freshwater diatom strain UTEX FD23 were revisited and its morphology studied for the first time. This strain is maintained at the UTEX Culture Collection of Algae, where it is referred to as Cocconeis placentula var. lineata. The sequences of this strain had been deposited in NCBI and used in more than 30 studies mostly under the name C. placentula, but also C. placentula var. lineata. We describe this species as new, based on sequence data and morphology (LM and SEM), which did not correspond to the epitypes of C. placentula var. placentula, C. lineata or any other described member of the C. placentula group. According to genetic distances for 18SV4, this new species is closest to C. placentula sensu stricto (5.8%), and has a distance of 7.9% to C. lineata. For rbcL, C. czarneckii is closest to the recently described C. coreana (2.6%-2.8%), it has a distance of 4.1% to C. lineata, and of 4.2% to C. placentula. Morphologically, C. czarneckii shares similarities with C. sijunghoensis, C. amerieuglypta, C. klamathensis and C. grovei, which differ by valve size and outline, numbers and pattern of submarginal areolae, and ultrastructural details of the valvocopula of the raphe valve. Our study, integrating available nucleotide sequences with a detailed morphological characterization of the strain UTEX FD23, added to our understanding of the heterogeneity of the C. placentula group, and will benefit the scientific community interested in this strain (e.g. for taxonomy, molecular phylogenetics and eDNA metabarcoding).