Thiodiglycol, or bis(2-hydroxyethyl)sulfide (also known as 2,2-thiodiethanol or TDE), is the organosulfur compound with the formula S(CH2CH2OH)2. It is miscible with water and polar organic solvents. It is a colorless liquid. It is structurally similar to diethylene glycol.

Thiodiglycol is manufactured by reaction of 2-chloroethanol with sodium sulfide.

Thiodiglycol is a polar protic solvent. It is used as a solvent in a variety of applications ranging from dyeing textiles to inks in some ballpoint pens. In chemical synthesis, it is used as a building block for protection products, dispersants, fibers, plasticizers, rubber accelerators, pesticides, dyes, and various other organic chemicals. In the manufacture of polymers, it is used as a chain transfer agent. As an antioxidant, it is used as an additive in lubricants.

Thiodiglycol is used as a mounting medium in microscopy. The ability to vary the refractive index of the medium by varying the concentration of TDE in an aqueous solution, plus its relative lack of toxicity makes it highly desirable for such use. The refractive index of the solution can be varied anywhere from near that of water (1.333) to that of glass (1.518).

Thiodiglycol is a Chemical Weapons Convention schedule 2 chemical used in the production of sulfur-based mustard gases. Thiodiglycol is also a product of the hydrolysis of the "mustard gas" bis(2-chloroethyl)sulfide and can be detected in the urine of casualties.