In this paper, a new Th4+ ion-selective chromogenic sensor (L) was developed by reacting 1,10-phenanthroline-2,9-dicarbohydrazide with 2-hydroxy naphthaldehyde. The sensing ability of L toward Th4+ was investigated in solution and paper strips loaded with L using spectrophotometric and colorimetric methods. The selective interaction of L was examined with various f-metal ions and other selected metal ions from s-block and d-block elements. Results show that by the colorimetric method in solution-phase dimethyl sulfoxide/H2O (7:3, v/v) and paper strip methods, the naked-eye detectable color change of L occurred from colorless solution to yellow-orange and pale yellow colour upon interacting with Th4+ and Al3+, respectively, whereas other metal ions did not interfere. The ligand L exhibits two absorbance bands at 320 and 375 nm because of ligand-to-ligand charge transfer. Upon interaction with Th4+, L undergoes red shift of both absorption bands and the formation of a new UV-vis band at 335 and 440 nm. The UV-visible spectral studies indicate the formation of a 1:1 host-guest complex between L and Th4+ with an association constant of 4.7 × 103 M-1. The limit of quantification and limit of detection of L for the analysis of Th4+ are found to be 167 and 50 nM, respectively. The visually detectable color change of L has been well integrated with a smartphone RGB color value to make it an analytical signal for real-time analysis of Th4+ with the detection limit down to 116 nM. Besides, L was applied for the analysis of Th4+ content present in various real water samples, monazite, and lantern mantle samples by spectrophotometry and RGB color values. The binding mode of L with Th4+ is investigated by 1H NMR, electrospray ionization-mass, and theoretical studies. Copyright © 2018 American Chemical Society.