Natural rubber (NR) is a sustainable biopolymer consisting mainly of cis-1,4-polyisoprene. Modifying an NR biosynthetic enzyme is a promising strategy to bioproduce novel polymers. Here, we have elucidated the NR biosynthetic mechanism and successfully developed novel enzymes that synthesise NR-sized polyisoprenes with unnatural substrates. NR is synthesised by a cis-prenyltransferase (cPT) on rubber particles (RPs), NR-harbouring lipid monolayer membrane organelles. However, the key to NR biosynthesis is not specialised cPTs, but the proper arrangement of cPTs on RPs since cPTs from various non-NR-producing organisms, such as humans, synthesise NR when introduced into the RPs. A tomato cPT, which condenses only one isoprene unit, was engineered to synthesise novel NR-sized polyisoprenes with artificial substrates by modifying residues for product size determination. Furthermore, the introduction of a modified trans-prenyltransferase into RPs led to the synthesis of NR-sized trans-1,4-polyisoprenes. This RP system could be used as a versatile platform for enzymatic polyisoprenoid synthesis.