In this work, single-crystalline gold nanoplates were produced by treating an aqueous solution of chloroauric acid with the extract of the unicellular green alga Chlorella vulgaris at room temperature. The results suggest proteins as the primary biomolecules involved in providing the dual function of Au(III) reduction and the size- and shape-controlled synthesis of the nanogold crystals. A protein with a molecular weight of approximately 28 kDa was isolated and purified by reversed-phase HPLC; this protein tested positive for the reduction of chloroauric acid in aqueous solution. The isolated protein (named gold shape-directing protein, or GSP for convenience) was then used to produce gold nanoplates with distinctive triangular and hexagonal shapes in high yields (approximately 90 %). The kinetics of the reduction reaction could be manipulated through changes in the GSP concentration to produce plates with lateral sizes ranging from nanometers to micrometers. The growth of gold nanoplates in the GSP solution with time was monitored by microscopic and spectroscopic techniques, thereby allowing the detection of several key intermediates in the growth process.