Integrated tasks are increasing in popularity, either replacing or complementing writing-only independent tasks in writing assessments. This shift has generated many research interests to investigate the underlying construct and features of integrated writing (IW) performances. However, due to the complexity of the IW construct, there are conflicting findings about whether and the extent to which various language skills and IW text features correlate to IW scores. To understand the construct of IW, we conducted a meta-analysis to synthesize correlation coefficients between scores of IW performances and (1) other language skills and (2) text quality features of IW. We also examined factors that may moderate the correlation of IW scores with these two groups of correlates. Consequently, (1) reading and writing skills showed stronger correlations than listening to IW scores; and (2) text length had a strongest correlation, followed by source integration, organization and syntactic complexity, with a smallest correlation of lexical complexity. Several IW task features affected the magnitude of correlations. The results supported the view that IW is an independent construct, albeit related, from other language skills and IW task features may affect the construct of IW.