In this article we describe the development of a part-of-speech (POS) tagger for Dutch messages from the Twitter microblogging website. Initially we developed a POS tag set ourselves with the intention of building a corresponding tagger from scratch. However, it turned out that the output of Frog, an existing high-quality POS tagger for Dutch, is of such quality that we decided to develop a conversion tool that modifies the output of Frog. The conversion consists of retokenization and adding Twitter-specific tags. Frog annotates Dutch texts with the extensive D-Coi POS tag set, which is used in several corpus annotation projects in the Netherlands. We evaluated the resulting automatic annotation against a manually annotated sub-set of tweets. The annotation of tweets in this sub-set have a high inter-annotator agreement and our extension of Frog shows an accuracy of around 95%. The add-on conversion tool that adds Twitter-specific tags to the output of Frog will be made available to other users.