Silver nanofillers of three different shapes were synthesized: silver nanospheres (AgNSs), silver nanowires (AgNWs) and silver nanowires–silver nanoplatelets (AgNWPs). Ultra-lightweight polyimide (PI) composite foams filled with these three silver nanofillers were fabricated by a facile and effective one-pot liquid foaming process, respectively. Their microstructure, electromagnetic interference (EMI) shielding effectiveness (SE) and shielding mechanisms were investigated. It was found that, at the same nanofiller loading, the EMI SE of the composite foams decreased in the following order: AgNWPs > AgNWs > AgNSs. AgNWPs/PI composite foams exhibited the highest EMI SE owing to the denser 3D conductive network of AgNWPs compared to AgNWs and AgNSs, in which the seamlessly interconnected AgNWPs network provided fast electron transport channels inside foams. Maximum specific EMI SE values of 1208 dB g⁻¹ cm³ at 200 MHz, 650 dB g⁻¹ cm³ at 600 MHz, and 488 dB g⁻¹ cm³ in the frequency ranges of 800–1500 MHz, 216–249 dB g⁻¹ cm³ at 8–12 GHz were achieved in the composite foams at 4.5 wt% AgNWPs loading, which far surpass the best values of other composite materials. The reflections of interconnected AgNWPs networks inside the foams combined with absorptions resulting from the multiple reflections at interfaces inside the foams contributed to the shielding effect. This suggests that our AgNWPs/PI composite foams have excellent potential as high-performance EMI shielding materials against electromagnetic interference pollution in applications that need lightweight.