The development and viability of seedlings are crucial in overwintering cereals, as it can affect not only an adult plant physiological condition, but also the size and quality of future grain. Recent studies on the genetic control of morphological and yield-related traits have been focused on adult plants of wheat, barley, maize, and rice. However, there is insufficient research describing the genetic control of cereals in the seedling stage. To fill that gap, the winter triticale ‘Hewo’ × ‘Magnat’-doubled haploid lines population was used to locate quantitative trait loci associated with selected traits measured at the seedling stage: the width and length of the first and second leaves, the leaf blade and leaf sheath lengths as well as the length-to-width ratio. Based on the genetic map for ‘Hewo’ × ‘Magnat’ DH population, main four loci, statistically significant (P < 0.05) and strongly associated with the second leaf length and width as well as their ratio were identified. Those regions were located on rye chromosomes 4R, 5R, and 6R with LOD value up to 20.35. In addition, candidate genes in localized regions were identified. The work also demonstrated correlations between the morphology of the second leaf, especially its length-to-width ratio, and the encoded proteins. As described in this paper, results are, to our knowledge, a first attempt to determine the genetic control of triticale seedlings and can provide a new insight on this field and become a reference for developing novel triticale varieties in breeding programmes.