In the plant RNA interference (RNAi) pathway, 21-nucleotide duplexes of small interfering RNA (siRNA) are processed from longer double-stranded RNA precursors by the RNaseIII Dicer-like 4 (DCL4). Single-stranded siRNAs then guide Argonaute 1 (AGO1) to execute posttranscriptional silencing of complementary target RNAs. RNAi is not cell-autonomous in higher plants, but the nature of the mobile nucleic acid(s) signal remains unknown. Using cell-specific rescue of DCL4 function and cell-specific inhibition of RNAi movement, we genetically establish that exogenous and endogenous siRNAs, as opposed to their precursor molecules, act as mobile silencing signals between plant cells. We further demonstrate physical movement of mechanically delivered, labeled siRNA duplexes that functionally recapitulate transgenic RNAi spread. Cell-to-cell movement is unlikely to involve AGO1-bound siRNA single strands, but instead likely involves siRNA duplexes.