An important element of learning to read and write at school is the ability to define word boundaries. Defining word boundaries in text writing is not a straightforward task even for children who have mastered graphophonemic correspondences. In children's writing, unconventional word segmentation has been observed across a range of languages and contexts with more occurrences of hyposegmentation (failure to separate two or more written words with a space) than hypersegmentation (written words are split into more than one segment). However, it is still unclear how frequent these errors are and the relationships of these written error patterns to the child's development in oral language, spelling and reading remains relatively unexplored. To address these issues, unconventional written lexical segmentations in Brazilian Portuguese children's text production during their first years at primary school (Year 1 to Year 3) were examined in relation to different cognitive and linguistic measures and patterns of spelling errors. Results reveal that in Portuguese the establishment of word boundaries in written text is not explained by visuospatial skills or limitations in processing resources (working memory). In contrast higher occurrences of hyposegmentation patterns were associated with lower levels of reading, vocabulary, verbal ability and morphological awareness whereas hypersegmentations were rarer and related to lower levels of reading and morphological awareness and typically only occurred in the initial stages of learning to write (Year 1). Occurrences of hyposegmentations as well as hypersegmentations were also related to spelling errors which reflected children's poor phonological skills.