Technosignature or technomarker is any measurable property or effect that provides scientific evidence of past or present technology. Technosignatures are analogous to biosignatures, which signal the presence of life, whether intelligent or not. Some authors prefer to exclude radio transmissions from the definition, but such restrictive usage is not widespread. Jill Tarter has proposed that the search for extraterrestrial intelligence (SETI) be renamed "the search for technosignatures". Various types of technosignatures, such as radiation leakage from megascale astroengineering installations such as Dyson spheres, the light from an extraterrestrial ecumenopolis, or Shkadov thrusters with the power to alter the orbits of stars around the Galactic Center, may be detectable with hypertelescopes. Some examples of technosignatures are described in Paul Davies's 2010 book The Eerie Silence, although the terms "technosignature" and "technomarker" do not appear in the book.

In February 2023, astronomers reported, after scanning 820 stars, the detection of 8 possible technosignatures for follow-up studies.

A Dyson sphere, constructed by life forms dwelling in proximity to a Sun-like star, would cause an increase in the amount of infrared radiation in the star system's emitted spectrum. Hence, Freeman Dyson selected the title "Search for Artificial Stellar Sources of Infrared Radiation" for his 1960 paper on the subject. SETI has adopted these assumptions in its search, looking for such "infrared heavy" spectra from solar analogs. Since 2005, Fermilab has conducted an ongoing survey for such spectra, analyzing data from the Infrared Astronomical Satellite.

Identifying one of the many infra-red sources as a Dyson sphere would require improved techniques for discriminating between a Dyson sphere and natural sources. Fermilab discovered 17 "ambiguous" candidates, of which four have been named "amusing but still questionable". Other searches also resulted in several candidates, which remain unconfirmed. In October 2012, astronomer Geoff Marcy, one of the pioneers of the search for extrasolar planets, was given a research grant to search data from the Kepler telescope, with the aim of detecting possible signs of Dyson spheres.

Shkadov thrusters, with the hypothetical ability to change the orbital paths of stars in order to avoid various dangers to life such as cold molecular clouds or cometary impacts, would also be detectable in a similar fashion to the transiting extrasolar planets searched by Kepler. Unlike planets, though, the thrusters would appear to abruptly stop over the surface of a star rather than crossing it completely, revealing their technological origin. In addition, evidence of targeted extrasolar asteroid mining may also reveal extraterrestrial intelligence (ETI). Furthermore, it has been suggested that information could be hidden within the transit signatures of other planets. Advanced civilizations could "cloak their presence, or deliberately broadcast it, through controlled laser emission". Other characteristics proposed as potential technosignatures (or starting points for detection of clearer signatures) include peculiar orbital periods such as arranging planets in prime number patterns. Coronal and chromospheric activity on stars might be altered. Extraterrestrial civilizations may use free-floating planets (rogue planets) for interstellar transportation with a number of proposed possible technosignatures.

A study suggests that if ETs exist, they may have established communications network(s) and may already have probes in the Solar System whose communication may be detectable. Studies by John Gertz suggest flyby (scout) probes might intermittently surveil nascent planetary systems and permanent probes would communicate with a home base, potentially using triggers and conditions such as detection of electromagnetic leakage or biosignatures. They also suggest several strategies to detecting local ET probes such as detecting emitted optical messages. He also finds that due to interstellar networks of communications nodes, the search for deliberate interstellar signals – as is common in SETI – may be futile. The architecture may consist of nodes separated by sub-light-year distances and strung out between neighboring stars. It may also contain pulsars as beacons or nodes whose beams are modulated by mechanisms that could be searched for. Moreover, a study suggests prior searches wouldn't have detected cost-effective electromagnetic signal beacons.

Various astronomers, including Avi Loeb of the Harvard-Smithsonian Center for Astrophysics. Edwin L. Turner of Princeton University, and Thomas Beatty of the University of Wisconsin have proposed that artificial light from extraterrestrial planets, such as that originating from cities, industries, and transport networks, could be detected and signal the presence of an advanced civilization.

Light and heat detected from planets must be distinguished from natural sources to conclusively prove the existence of intelligent life on a planet. For example, NASA's 2012 Black Marble experiment showed that significant stable light and heat sources on Earth, such as chronic wildfires in arid Western Australia, originate from uninhabited areas and are naturally occurring.