A mercury-in-glass thermometer is a thermometer which provides temperature readings through the expansion and contraction of mercury inside a calibrated tube. These devices were used extensively to measure temperatures in a wide variety of settings from the 1700s until the 20th century, when concerns about the health risks of mercury exposure led many nations to place restrictions on mercury-in-glass thermometers. Medical use of these instruments is largely banned, with remaining manufacture of mercury-in-glass thermometers being aimed primarily at meteorological researchers.

These devices work by having a large bulb filled with mercury in the bottom of the thermometer, attached to a thin tube. As the temperature rises, the mercury expands, traveling up the tube. People can read the temperature with a mercury-in-glass thermometer by finding the marking which correlates with the height of the mercury. As the temperature lowers, the mercury contracts, shrinking back down into the bulb. In extremely cold temperatures, the mercury-in-glass thermometer will not function, because the mercury itself will be frozen. With a maximum thermometer, the mercury will be held at the highest point until the thermometer is shaken, allowing someone to read the maximum temperature registered on the thermometer.

The developers of the Fahrenheit and Celsius scales both used the mercury-in-glass thermometer in their research, and created a system of calibration which allowed for the development of standardized temperature scales. To calibrate a mercury-in-glass thermometer, people use known temperature conditions, such as boiling water, to confirm that the thermometer is reading correctly.

The design of this thermometer is not intended to release mercury into the environment. In fact, a very closely sealed environment is created inside the thermometer to avoid malfunction. However, when such thermometers break or are discarded, there is a risk of environmental contamination from the escaping mercury. While the amount of mercury in a single mercury-in-glass thermometer is not generally viewed as a major health risk, the collective contributions of scores of broken and discarded thermometers could be significant.

There are a number of methods which can be used to create an accurate thermometer, creating an array of alternatives to the mercury-in-glass thermometer. For medical use, people tend to prefer safer options to avoid exposing people to unnecessary health risks. Certain scientific researchers may prefer to work with mercury-in-glass thermometers for various reasons, although care is taken with the handling and disposal of such thermometers, in response to health concerns and environmental laws.