It shouldn't be too surprising that eventually somebody decided to combine the two.
Compare Transhuman Aliens, for when transhumans are substituted for aliens, Genetic Adaptation, for when humans adapt themselves to new environments, and Space People, for when humans adapt themselves to live in outer space.
While Transhuman Space certainly qualifies, they are not to be confused.
- Knights of Sidonia is a fairly standard Space Opera of a colony ship of humans fleeing the destruction of Earth by alien creatures known as the Gauna, but being written by Tsutomu Nihei, transhumanism is rampant to the point where there are no baseline human characters. A short list of transhuman features present in the series are human capable of the photosynthesizing to reduce (but not eliminate) the need for food (everyone except the main character), medically induced immortality (the command crew), an enhanced Healing Factor implied to be tied to an innate form of biological immortality (main character Tanikaze), cybernetic replacements for lost limbs (Lalah and later Isana), an artificially created third sex that eventually transitions to either male or female after becoming attracted to someone (Isana and a couple of minor characters), mass cloning (the Honoka sisters), Super-Strength (again, the Honoka sisters). That doesn't even begin to cover some of the more extreme instances in the series courtesy of Ochiai.
- In Strange Frame, humans have moved out to the Jovian belt. In order to survive in the different environments of the Jovian moons, most humans have undergone some degree of genetic or cybernetic modification.
- Aeon 14: Cyborgs, Bio-Augmentation, and fully self-aware Artificial Intelligences are all over the setting, up to and including known cases of AI merging with humans and organics and AI ascending beyond flesh and silicon. At the same time though, they still have recognizably human personalities, behaviors and motivations, and love, hate, fight, laugh and cry.
- Blindsight has a team of transhumans sent to the Oort Cloud to make First Contact. The entire crew have hibernation genes from prehistoric vampires, the biologists have cybernetic senses at the expense of expressive control, the linguist has four artificially induced split personalities, the combat specialist is wired into a team of drones, the captain is an actual vampire, and the protagonist had a radical hemispherectomy to cure his epilepsy, so he has a prosthetic brain hemisphere.
- Crest of the Stars has the Abh, a race of humans with genetically engineered modifications to better withstand space travel (and also for cosmetic reasons). Even more heavily modified groups exist, but they are generally fairly reclusive. The only non-Abh modified character we meet is a United Mankind intelligence agent who is descended from a group that modified themselves for extended lifespans.
- The Culture is a civilization spanning a good chunk of the galaxy whose citizens engage in frequent body-modding ranging from drug glands to extra limbs to brain backups.
- Eldraeverse: The Eldrae were transhuman to begin with, being Transplanted Humans genetically modified by Precursors. Around the time they started venturing out into space, they were taking things further, such as adding Body Backup Drive to their "natural" lack of aging.
- Gregory Benford's Galactic Center saga is an early example, written starting in 1977. In the first two books, humans use technology salvaged from hostile aliens that attacked the solar system to travel to the center of the galaxy at near light speed. By the third book, their descendants have become post-human cyborgs who rapidly swap out cybernetic implants and can copy much of their memories onto computer chips, despite having lost nearly all of their knowledge of history.
- Great Ship: Humanity, who for the most part have started replacing their brains with ceramic hardware, was the first species to gain access to the titular massive starship.
- Incandescence has a combination of Body Uploading and Destructive Teleportation. If someone moves from the physical world to a processor, their body is destroyed when their mind is digitized. If they want to take on a physical body again, computers design one to their specifications. There are also nodes in interstellar space where people live full-time on a processor. At the start of the book, Rakesh hasn't been in the physical world in 96 years, and his friend Parantham was created de novo on a processor and never had a body to begin with.
- Moon Rainbow concerns the emergence of the first transhumans ("exotes") during humanity's initial exploration of the Solar system and their subsequent exile from Earth to a nearby star system. After a massive Time Skip, the last published novel sees exotes allowed back on Earth in return for their help in exploring the wider galaxy, since regular humans cannot pilot Faster-Than-Light Travel spaceships, which the exotes have invented during their exile.
- Old Man's War: The human Colonial Defense Forces go all out with their engineered Super-Soldier bodies — super-strength, super-senses, enhanced reflexes, green skin, fully synthetic blood. They have to, in order to stand a chance against other species more blessed by evolution.
- Schismatrix portrays a spacefaring setting dominated by transhumans and split between shapers who favor biotech and mechanists who favor cybernetics. Characters routinely and casually receive bionic and biological enhancements, especially in short fiction set in the shaper/mechanist 'verse.
- In the Takeshi Kovacs series, Earth humans have colonized several star systems by uploading their brains to cloned bodies, often with biological or cybernetic enhancements.
- In That Hideous Strength, N.I.C.E. hopes to bring the vision of Out of the Silent Planet's villain to life by creating an interplanetary empire. Unlike him, they have also set their sight on eliminating the flaws of organic matter and hope to replace humanity with a select few minds that have been stripped of their flesh, allowing them to eventually create a totally hygienic and controlled universe.
- Blue Planet has genetically and cybernetically modified humans and uplifts exploring an oceanic planet.
- Eclipse Phase: During a Singularity gone wrong, Earth was evacuated by Brain Uploading to artificial bodies produced throughout the solar system.
- Hc Svnt Dracones: After a nuclear war on Earth the corporate colonies on Mars repopulated the solar system with genetically engineered human-animal hybrids and Ridiculously Human Robots. 700 years later there are entire Mega Corps specializing in Bio or Cyber augmentation.
- Mindjammer: Ten thousand years after launching STL colony ships, Earth develops FTL travel and the planet's immortal oligarchs reach out to the various branches of humanity scattered throughout the stars.
- Nova Praxis: A seed AI known as "Mimir" provided the nanotechnology both to convert a human brain into a synthetic neural network and to build stargates.
- Transhuman Space takes place in the year 2100 in a solar system colonized by genetically engineered humans, Artificial Humans, and AI.
- Civilization: Beyond Earth: Most of the Affinities use some variety of this concept when Settling the Frontier of alien worlds. In those cases, the Affinity level indicates how far the faction has diverged from the human baseline.
- Supremacy uses Cyborg enhancements, Artificial Intelligence, and eventual Brain Uploading to develop a civilization that can flourish irrespective of conditions on alien worlds. Its unique victory option returns to Earth to share its technology-born immortality in what might be a Benevolent Alien Invasion, an Assimilation Plot, or both.
- Harmony uses Bio-Augmentation and Genetic Adaptation to adapt to alien environments, ultimately becoming part of the alien ecosystem. Its unique victory option merges all life on the planet into a single transcendent consciousness.
- The Harmony-Purity hybrid Affinity believes that Humanity Is Superior and engineers Designer Babies in an attempt to refine the human race into an Ultimate Life Form.
- The Supremacy-Harmony hybrid Affinity uses both biological and technological enhancements so enthusiastically that it can become nigh-impossible to distinguish flesh from machine, never mind tell whether the organism was human to begin with.
- In RimWorld, humans across the galaxy can have bionic body parts and brain implants, and with rare nanotech therapies, regenerate missing parts, learn new skills, or even come back from the dead. In the expansions, there are a variety of people vats and bioengineered subspecies.
- Always Human: Bio-Augmentation "mods" are ubiquitous on Earth, humans are beginning to colonize the solar system, and space travelers get a suite of specialized mods to acclimate themselves to off-world conditions as a matter of course.
- There's a lot more in the way of Genetic Adaptation in the part of the Freefall universe we see, but there's also a major focus on robots, uplifting, and how AI can be just as human as anyone else. (Also, Sam, whose Blue-and-Orange Morality is played for laughs — and turns out to be a lot more human than expected.) Different characters have different viewpoints, and Qwerty and Dvorak are basically synthetic transhumanists. The biggest transhumanist we see, later on, is Tess Thurmad, whose views are basically, 'let's barrel forward, full speed ahead!'
- Quantum Vibe is a romp across a Colonized Solar System where cortical implants are as common as smartphones and life-extension is common, at least in the prosperous regions. The characters encounter Human Subspecies like the multi-armed Spyders, the super-strong Belt Apes, and (most heartbreakingly) the Fantastic Caste System of West Terra. After the Time Skip, Brain Uploading into android bodies becomes common.
- In Runaway to the Stars, humanity stands out among explored space's five known sapient species by their predilection for genetically engineering themselves. There are dozens of "clades" of genetically modified humans (GMHs) and many more one-off creations.
- Schlock Mercenary: The military characters host Boring, but Practical "soldier-boost" nanotech, which gets increasingly advanced over the series, to the point of armor-piercing claws, secreted weapons, growing an armored carapace, and cheating death as a Body Backup Drive. One human character chooses to keep her carapace, even in civilian life as a scientist. Many characters are Uplifted Animals and humans with unusual skin colors.
- Orion's Arm is a future history where, over the course of the next ten millennia, humanity branches off into millions of modified clades, creates countless varieties of AI, and uplifts seemingly every species on Earth and many other planets within a 5,000-light-year radius of Sol.