The influence of poly(tetrafluoroethylene) (PTFE) nanofibers, generated in situ during shear extrusion, on foaming behavior of linear (L-PP) and long-chain branched (LCB-PP) polypropylenes has been investigated. PTFE nanofibers significantly improved both thermal and rheological properties of PPs studied. PTFE nanofibers formed an entangled network and induced the strain hardening of PP/PTFE nanocomposites. Foams of neat PP and PP/PTFE nanocomposites were produced using continuous extrusion foaming and autoclave-based bead foaming. The presence of PTFE nanofibers resulted in much smaller foam cells and higher cell concentration as compared to neat PPs. In addition, the autoclave-based bead foaming showed that PTFE nanofibers nucleated cells formation in LCB-PP and participated in controlling the cells growth. The entanglements between PTFE nanofibers prevented excessive expansion of cells. As the result, expanded beads of LCB-PP/PTFE nanocomposite possessed ten times smaller cells, narrower cell size distribution and much higher cell concentration as compared to neat LCB-PP.