In the title Mannich base, C(20)H(21)N(3)O(3), an isatin derivative of thymol, the O-CH(2)-C(=O)-N(H)-N fragment connect-ing the aromatic and fused-ring systems is approximately planar, with the N-N single bond in a Z configuration. The amino H atom of this N-N fragment is intra-molecularly hydrogen bonded to the carbonyl O atom of the indolinone fused ring as well as to the phen-oxy O atom of the aromatic ring. The amino H atom of the indoline fused ring forms a hydrogen bond with the double-bond O atom of an adjacent mol-ecule, this hydrogen bond giving rise to a linear chain motif.