The 5-volume Proceedings, Lnai 12457 Until 12461 Constitutes The Refereed Proceedings Of The European Conference On Machine Learning And Knowledge Discovery In Databases, Ecml Pkdd 2020, Which Was Held During September 14-18, 2020. The Conference Was Planned To Take Place In Ghent, Belgium, But Had To Change To An Online Format Due To The Covid-19 Pandemic.the 232 Full Papers And 10 Demo Papers Presented In This Volume Were Carefully Reviewed And Selected For Inclusion In The Proceedings. The Volumes Are Organized In Topical Sections As Follows: Part I: Pattern Mining; Clustering; Privacy And Fairness; (social) Network Analysis And Computational Social Science; Dimensionality Reduction And Autoencoders; Domain Adaptation; Sketching, Sampling, And Binary Projections; Graphical Models And Causality; (spatio-) Temporal Data And Recurrent Neural Networks; Collaborative Filtering And Matrix Completion.