A new all-Solid-state ion selective electrode (ASS-ISE) was developed for the determination of thulium trivalent cation. The potentiometric sensor was made by depositing a thin PVC membrane containing N՛-[(2-hydroxyphenyl)methylidene]-2-furohydrazide (L) as a selectophore on a conducting Solid-state electrode. The Solid-state electrode was a thin copper wire coated by a mixture of multi-walled carbon nanotubes (MWCNTs) and an epoxy resin. The optimal composition of the PVC membrane was 25%wt. of PVC powder, 68% wt. of benzyl acetate, 2% wt. of sodium tetraphenyl borate (NaTPB)), and 5% wt. of L. The sensor showed good selectivity for Tm3+ ion with a Nernstian calibration plot slope of 19.6±0.4 mV/decade in Tm3+ ion solutions with concentrations ranging from 1.0×10-8 M to 1.0×10-3 M, and its limit of detection was as low as 6.3×10-9 M. The proposed sensor was able to detect Tm3+ cations in the wastewater matrix.