A thermowell adapter secures a sheathed and flanged sensor within an internally threaded thermowell. The thermowell adapter has a head and a body extending from the head. The body of the thermowell adapter has external threading for mating with the internally threaded thermowell. A passageway within the head and body defines an axis coaxially with the external threading. The sheathed and flanged probe extends within the passageway such that a sensing element of the sheathed and flanged probe extends into the thermowell. An axial securement on the head prevents the sheathed and flanged probe from moving axially in the passageway. A rotational securement that is separate from the axial securement interacts in a mating relationship with squeeze locations on the sheathed and flanged probe to prevent the probe from rotating about its axis in the passageway.