Synthetic macrocycles show great promise as therapeutics because they can target complex biological structures with exceptional specificity and potency. However, macrocycle screening methods such as phage or mRNA display depend on molecular biology, which inherently limits the scope of chemical diversity available for exploration and the range of druggable targets. Here, we report CycloSEL - a barcode-free, fully synthetic ultra-high throughput macrocycle discovery platform. CycloSEL integrates combinatorial synthesis of massive libraries, affinity selection to isolate potent binders, and barcode free decoding using de novo tandem mass spectrometry. We performed affinity selections with ~ 16 million macrocycles against the oncology target Carbonic Anhydrase IX, achieving excellent enrichment of true binders with low nanomolar affinities. We also screened the library against WDR5 to identify inhibitors that block its interaction with rearranged MLL, a key driver of acute myeloid leukaemia. From this, we discovered a metabolically stable and cell permeable macrocycle with an 8 nM affinity that potently disrupts the WDR5-MLL interaction. Overall, CycloSEL is a powerful barcode-free screening platform for massive synthetic macrocycle libraries, offering tremendous potential for drug discovery efforts aiming at challenging disease targets.