These are the proceedings of the International Workshop on Petri Nets and Software Engineering (PNSE'2021) to be held at June 25, 2021. The workshop is co-located with 42nd International Conference on Application and Theory of Petri Nets and Concurrency (PETRI NETS 2021) in Paris, France. The workshop emphasizes on the impact of software engineering in general and its development based on Petri nets. Therefore we have especially invited contributions that work on subjects that could be relevant for Petri nets, their application, supporting tools, etc. Results on other formalisms or semi-formal techniques are highly welcome. More information about the workshop can be found at: http://www.pnse.de/ The previous edition of PNSE in 2020 should also have been in Paris, but due to the pandemic it has to be be organised as a virtual meeting. By the time this workshop was organised we all hoped that it would be possible to meet in Paris again. We were too optimistic. Overall we received 20 contributions for these proceedings. Each paper was reviewed by at least three referees. During the reviewing process the program comittee selected 13 contributions for publication. We are glad that Joel Greenyer accepted our invitation to give an inivted talk about Scenario-Based Modeling and Programming of Distributed Systems. The international program committee of PNSE'21 was supported by the valued work of Artur Meski, Teo l Sidoruk, Mitchell Klijs, Malte Heitho , and Joel Charles as additional reviewers. Their valuable work is highly appreciated. Furthermore, we would like to thank our colleagues in the PC and the local organization team for their support { we are all now `experts' in the organisation of virtual events.