He is best known for his science fiction novels set in the Humanx Commonwealth, an interstellar ethical/political union of species including humankind and the insectoid Thranx. Many of these novels feature Philip Lynx , an empathic young man who has found himself involved in something which threatens the survival of the Galaxy. Flinx's constant companion since childhood is a minidrag named Pip, a flying, empathic snake capable of spitting a highly corrosive and violently neurotoxic venom.

Foster's best-known fantasy work is the Spellsinger series, in which a young musician is summoned into a world populated by talking creatures where his music allows him to do real magic whose effects depends on the lyrics of the popular songs he sings (although with somewhat unpredictable results).

Many of Foster's works have a strong ecological element to them, often with an environmental twist. Often the villains in his stories experience their downfall because of a lack of respect for other alien species or seemingly innocuous bits of their surroundings. This can be seen in such works as Midworld, about a semi-sentient planet that is essentially one large rainforest, and Cachalot, set on an ocean world populated by sentient cetaceans. Foster usually devotes a large part of his novels to descriptions of the strange environments of alien worlds and the coexistence of their flora and fauna. Perhaps the most extreme example of this is Sentenced to Prism, in which the protagonist finds himself trapped on a world where life is based on silicon rather than carbon, as on Earth.

Foster has been so prolific that he is often rumored to have been the ghostwriter on novels with which he had little direct involvement, such as the novelization of The Motion Picture, which was credited to (and actually written by) Gene Roddenberry. Foster wrote the treatment on which the film was based, perhaps accounting for the misattribution of the novel to him. He also authored 10 volumes of novelizations based upon The Animated Series, several of which involving taking the script for a half-hour episode and expanding it into a full-length novel. He later wrote the novelization of the 2009 film Star Trek, his first Star Trek novel in over 30 years.

It has long been known that Foster co-wrote the original novelization of Star Wars (later retitled Star Wars Episode IV: A New Hope) which had been credited solely to George Lucas. When asked if it was difficult for him to see Lucas get all the credit for Star Wars, Foster said "Not at all. It was George's story. I was merely expanding upon it. Not having my name on the cover didn't bother me in the least. It would be akin to a contractor demanding to have his name on a Frank Lloyd Wright house."

Commonwealth novels

• Midworld (1975), ISBN 0-345-35011-1
• Cachalot (1980), ISBN 0-345-28066-0
• Nor Crystal Tears (1982), ISBN 0-345-29141-7
• Voyage to the City of the Dead (1984), ISBN 0-345-31215-5
• Sentenced to Prism (1985), ISBN 0-345-31980-X
• The Howling Stones (1997), ISBN 0-345-38375-3
• Drowning World (2003), ISBN 0-345-45035-3
• Quofum (2008), ISBN 978-0-345-49605-8

Novels featuring Pip and Flinx

• For Love of Mother-Not (1983), ISBN 0-345-30511-6
• The Tar-Aiym Krang (1972), ISBN 0-345-29232-4
• Orphan Star (1977), ISBN 0-345-25507-0
• The End of the Matter (1977), ISBN 0-345-25861-4
• Flinx in Flux (1988), ISBN 0-345-34363-8
• Mid-Flinx (1995), ISBN 0-345-38374-5
• Reunion (2001), ISBN 0-345-41867-0
• Flinx's Folly (2003), ISBN 0-345-45038-8
• Sliding Scales (2004), ISBN 0-345-46156-8
• Running from the Deity (2005), ISBN 0-345-46159-2
• Bloodhype (1973), ISBN 0-345-25845-2
• Trouble Magnet (2006), ISBN 0-345-48504-1
• Patrimony (2007), ISBN 978-0-345-48507-6
• Flinx Transcendent (2009), ISBN 978-0345496072

Icerigger trilogy

• Icerigger (1974), ISBN 0-345-23836-2
• Mission to Moulokin (1979), ISBN 0-345-27676-0
• The Deluge Drivers (1987), ISBN 0-345-33330-6

Founding of the Commonwealth

• Phylogenesis (1999), ISBN 0-345-41862-X
• Dirge (2000), ISBN 0-345-41864-6
• Diuturnity's Dawn (2002), ISBN 0-345-41865-4

Media novelizations

• Luana (Italian film) (1974), ISBN 0-345-23793-5
• Dark Star (1974), ISBN 0-345-24267-X
• Alien (1979), ISBN 0-446-82977-3
• Aliens (1986), ISBN 0-446-30139-6
• Alien 3 (1992), ISBN 0-446-36216-6
• The Black Hole (1979), ISBN 0-345-28538-7
• Clash of the Titans (1981), ISBN 0-446-93675-8
• Outland (1981), ISBN 0-446-95829-8
• The Thing (1981), ISBN 0-553-20477-7
• Krull (1983), ISBN 0-446-30642-8
• The Last Starfighter (1984), ISBN 0-425-07255-X
• Starman (1984), ISBN 0-446-32598-8
• Shadowkeep (1984 computer game) (also a novel) (1984), ISBN 0-446-32553-8
• Pale Rider (1985), ISBN 0-446-32767-0
• Alien Nation (1988), ISBN 0-446-35264-0
• The Dig (computer game) (also a novel) (1995), ISBN 0-446-51853-0
• The Chronicles of Riddick (2004), ISBN 0-345-46839-2
• Star Trek (2009), ISBN 1-4391-5886-X
• Terminator Salvation (2009), ISBN 1-8485-6085-0

Transformers

• Transformers (2007), ISBN 0-3454-9799-6
• Ghosts of Yesterday (2007), ISBN 0-3454-9798-8
• Revenge of the Fallen (2009), ISBN 978-0-345-51593-3
• The Veiled Threat (2009), ISBN 0-3455-1592-7

Star Trek: The Animated Series

• Star Trek Log 1 (1974), ISBN 0-345-24014-6
• Star Trek Log 2 (1974), ISBN 0-345-25812-6
• Star Trek Log 3 (1975), ISBN 0-345-24260-2
• Star Trek Log 4 (1975), ISBN 0-345-24435-4
• Star Trek Log 5 (1975), ISBN 0-345-33351-9
• Star Trek Log 6 (1976), ISBN 0-345-24655-1
• Star Trek Log 7 (1976), ISBN 0-345-24965-8
• Star Trek Log 8 (1976), ISBN 0-345-25141-5
• Star Trek Log 9 (1977), ISBN 0-345-25557-7
• Star Trek Log 10 (1978), ISBN 0-345-27212-9
• Star Trek: The Motion Picture Photostory (novelization of The Motion Picture) (1980), ISBN 0-671-83089-9

Star Wars stories

• From the Adventures of Luke Skywalker (novelization of Star Wars; as George Lucas) (1976), ISBN 0-345-26079-1
• Splinter of the Mind's Eye (1978), ISBN 0-345-27566-7
• The Approaching Storm (2002), ISBN 0-345-44300-4

The Damned Trilogy

• A Call to Arms (1991), ISBN 0-345-35855-4
• The False Mirror (1992), ISBN 0-345-35856-2
• The Spoils of War (1993), ISBN 0-345-35857-0

Spellsinger

• Spellsinger (1983), ISBN 0-446-97352-1
• The Hour of the Gate (1984), ISBN 0-446-90354-X
• The Day of the Dissonance (1984), ISBN 0-446-32133-8
• The Moment of the Magician (1984), ISBN 0-446-32326-8
• The Paths of the Perambulator (1985), ISBN 0-446-32679-8
• The Time of the Transference (1986), ISBN 0-932096-43-3
• Son of Spellsinger (1993), ISBN 0-446-36257-3
• Chorus Skating (1994), ISBN 0-446-36237-9

Dinotopia

• Dinotopia Lost (1996), ISBN 1-57036-279-3
• The Hand of Dinotopia (1997), ISBN 1-57036-396-X

Journeys of the Catechist

• Carnivores of Light and Darkness (1998), ISBN 0-446-52132-9
• Into the Thinking Kingdoms (1999), ISBN 0-446-52136-1
• A Triumph of Souls (2000), ISBN 0-446-52218-X

The Taken trilogy

• Lost and Found (2004), ISBN 0-345-46125-8
• The Light-Years Beneath My Feet (2005), ISBN 0-345-46128-2
• The Candle of Distant Earth (2005), ISBN 0-345-46131-2

Story collections

• With Friends Like These (1977), ISBN 0-345-25701-4
• Who Needs Enemies? (1984), ISBN 0-345-31657-6
• The Metrognome and Other Stories (1990), ISBN 0-345-36356-6
• Montezuma Strip (1995), ISBN 0-446-60207-8
• Mad Amos (1996), ISBN 0-345-39362-7
• Impossible Places (2002), ISBN 0-345-45041-8
• Exceptions to Reality (2008), ISBN 0-345-49604-3

Other books

• The Man Who Used the Universe (1983), ISBN 0-446-90353-1
• The I Inside (1984), ISBN 0-446-32027-7
• Slipt (1984), ISBN 0-425-07006-9
• Into the Out Of (1986), ISBN 0-446-51337-7
• Glory Lane (1987), ISBN 0-441-51664-5
• Maori (1988), ISBN 0-441-51925-3
• Outer Heat (1988), ISBN 0-446-35265-9
• To the Vanishing Point (1988), ISBN 0-446-51338-5
• Quozl (1989), ISBN 0-441-69454-3
• Cyber Way (1990), ISBN 0-441-13245-6
• Cat-a-lyst (1991), ISBN 0-441-64661-1
• Codgerspace (1992), ISBN 0-441-71851-5
• Greenthieves (1994), ISBN 0-441-00104-1
• Life Form (1995), ISBN 0-441-00218-8
• Design for Great-Day (with Eric Frank Russell) (1995), ISBN 0-312-85501-X
• Jed the Dead (1997), ISBN 0-441-00399-0
• Parallelities (1998), ISBN 0-345-42461-1
• Interlopers (2001), ISBN 0-441-00847-X
• Kingdoms of Light (2001), ISBN 0-446-52667-3
• Primal Shadows (2001), ISBN 0-312-87771-4
• The Mocking Program (2002), ISBN 0-446-52774-2
• Sagramanda (2006), ISBN 1-59102-488-9

Unpublished

• Maude (1974)

Foster won the 2008 Grand Master award from the International Association of Media Tie-In Writers.