Protein nanofibrils (PNFs), especially those from the whey protein β-lactoglobulin, hold the promise for applications in food technology, medicine, and sustainable materials. In this work, we explore the mechanisms underlying the sol-gel transition of whey protein isolate triggered by pre-fragmented whey fibrils (seeds) at low pH and high temperature. We show that, under these conditions, the formed hydrogels are constructed from PNFs. The presented results suggest that the seeds amplify the fibril growth process by providing active ends that capture peptide monomers produced via acid hydrolysis. This changes the fibrils' length distribution (up to 10-fold increase of their average contour length), and the samples reach the percolation threshold at a much lower mass concentration of fibrils. We also note that seeding has a strong impact on morphology and catalyzes a conversion of short, curved fibrils into long straight ones, which also contribute to the lower percolation limit. Rheological measurements indicate that attractive inter-fibrillar forces stabilize the PNF network. This is further evidenced by the gels’ resistance to disassembly across a wide pH range, implying that other forces than electrostatics are important for stabilizing the fibrillar network. Finally, we discuss the nature of the sol-gel transition based on continuum percolation theory, which corroborates the observed relationship between PNF length distribution and the sol-gel transition.