Background: Phonological processing skills have been found to contribute to spelling development across different orthographies; however, less is known about the role of orthographic knowledge. This longitudinal study explores the contribution of phonological and orthographic knowledge to spelling development in a semi-transparent orthography (Swedish) across a period of 10 years. Methods: A group of Swedish speaking children were assessed on phonological recoding (phonological choice-task), orthographic knowledge (choice-task) and spelling (dictation task) in primary school (grade 2, age 8, total N = 99), secondary school (grade 8, age 14, N = 99) and again in upper secondary school (year 2, age 17, N = 79). Furthermore, spelling in a natural writing assignment was collected in upper secondary school. Spelling scores from grade 8 (dictation) and year 2 in upper secondary school (dictation and text) were included as dependent variables in three sets of hierarchical regression analyses. In the first step spelling performance in grade 2 was included to control for the autoregressive effect. In the second step, orthographic knowledge and phonological recoding from grade 2 were entered into the model in order to test for the longitudinal prediction. Results: Test scores within and across ages were significantly correlated in bivariate analysis. Regression analysis revealed that orthographic knowledge in grade 2 was a unique longitudinal predictor of spelling performance across time-points (secondary and upper secondary school) and assessment formats (dictation and text), beyond the contribution of the control variables. Conclusions: This study confirms the role of early orthographic knowledge in Swedish spelling development throughout the school years assessed in standardized dictation tasks as well as in naturalistic writing assignments.