This study examines sustainability reporting assurance (SRA) provider use of sustainability restatements as a means to create legitimacy in the developing SRA market. In comparison to financial data, mistakes in sustainability reporting are more likely to be made and less likely to be discovered prior to reporting. A lack of clear reporting standards and ambiguous SRA guidelines create a setting where providers can use restatements in an attempt to demonstrate both a problem in sustainability reporting and assurance as the solution to that issue. Based on a sample of US firms from 2010 to 2014, we find that SRA is associated with an increased likelihood of sustainability restatements, that the association is stronger for error restatements than for restatements due to methodological updates, and that SRA is significantly associated with the disclosure of quantitatively non-material restatements. We also document differences in these relations across provider type, with only consultant assurance significantly associated with methodological restatements and restatements of a non-material amount. Our findings support differences between sustainability report restatements and financial restatements, and provide evidence in support of our argument that assurance providers may be using restatements in an attempt to expand market share in a new professional space.