Human cytomegalovirus (HCMV) is a ubiquitous pathogen that infects the majority of the adult population. Similarly to other herpesvirus, it is able to enter a state of latency, persisting within the host for life. Infection of healthy individuals is normally asymptomatic, however it is among the most common causes of allograft rejection, and can lead to several life-threatening diseases in the immunocompromised. Moreover, it is the leading viral infectious cause for congenital disease. HCMV encodes 171 canonical genes, and a substantial number of non-canonical ORFs have been identified by ribosome profiling and proteomics. The functions of many canonical HCMV proteins remain poorly understood, and it is not yet clear how many non-canonical ORFs encode functional polypeptides. Recent studies have provided an extensive overview on the modulation of gene expression as well as the spatio-temporal dynamics of viral and host proteins during HCMV infection. However, characterisation of specific protein-protein interactions and the exact molecular mechanisms underpinning the biological changes observed during viral infection are beyond the scope of these approaches. Affinity-purification mass spectrometry was performed to identify binding partners for 169 canonical, and 2 non-canonical HCMV proteins in infected cells. CompPass filtering determined an extensive network of high-confidence interacting proteins, with >3,400 virus-host and >150 virus-virus protein interactions. Domain association analysis identified protein domains co-occurring with unusual frequency, while functional enrichment analysis provided an insight into novel functions of multiple viral genes as well as how HCMV systematically modulates host environment, for example interacting with transcriptional repressive complexes or families of ubiquitin E3 ligases. Furthermore, combining interaction data with a recently published systematic analysis of HCMV-induced protein degradation identified viral interactors for 31/133 degraded host targets. Finally, the uncharacterised, non-canonical ORFL147C protein was found to interact with elements of the mRNA splicing machinery, and a mutational study suggested its importance in viral replication. The interactome data will be important for future studies of herpesvirus infection.