The complement molecule C5a is a core effector of complement activation. C5a exerts potent pro-inflammatory and immunomodulatory actions via the C5a receptors C5aR1 and C5aR2. Besides complement, innate immune recognition of pathogens and sterile insults is also achieved by pattern-recognition receptors (PRRs). PRRs recognise conserved molecules or structural motifs, namely pathogen-associated molecular patters and danger-associated molecular patterns (DAMPs). Whilst C5a receptor-PRR cross-talk has been studied in the context of the toll-like receptors (1), the potential interactions between C5a and non-toll PRRs remain underexplored. The PRR Dectin-1, is a C-type lectin receptor that recognises fungal β-glucans and DAMPs, playing an important role in host defence. Considering prior interactions between C5a and Dectin-1 demonstrated in murine neutrophils (2), here we aimed to investigate how C5a, acting through C5aR1 and/or C5aR2, might modulate Dectin-1-mediated cytokine responses in primary human monocyte-derived macrophages (HMDMs). In line with other studies, C5a alone did not stimulate cytokine production from macrophages. However, when co-treating HMDMs with the selective Dectin-1 activator, depleted-zymosan, we observed a concentration-dependent, bidirectional action of C5a. Low concentrations (1 nM) of human C5a significantly dampened Dectin-1-mediated TNFα release (by ~40%), whilst a high concentration (100 nM) of C5a was strongly upregulatory (> 300% increase). Inhibiting C5aR1 with the selective antagonists PMX53 and JPE1375 significantly dampened this synergistic effect by ~50%, suggesting a dual role for both C5aR1 and C5aR2 in this process. In addition to direct co-treatment, priming HMDMs with C5a prior to depleted-zymosan also enhanced Dectin-1-induced TNFα, albeit to a smaller extent, indicating that C5a receptor signalling can induce this activity separate to the timing of Dectin-1 signalling. Mechanistically, inhibiting Syk kinase, reactive oxygen species, and NFκB significantly dampened both the Dectin-1 alone and Dectin-1+C5a induced TNFα. This study demonstrates a powerful synergistic effect of human C5a on Dectin-1 mediated cytokine responses in human macrophages, further highlighting the presence of an intricate cross-talk between complement and pattern-recognition systems in driving innate immune responses.