A Door into Ocean is a 1986 feminist science fiction novel by Joan Slonczewski. It is the first book in Slonczewski's Elysium Cycle. The novel's themes include ecofeminism and nonviolent revolution, as well as Slonczewski's own knowledge in the field of biology.

The novel is set in the future, on the fictional planets of Shora and Valedon. Shora is a moon of Valedon, covered by water. The inhabitants of this planet, known as Sharers, are all female. Sharers use genetic engineering to control the ecology of their planet. They are peaceful beings who "share" — that is, they have a spiritual and linguistic union with each other and treat everyone equally. Valedon, by contrast, is one of many planets ruled by a transhuman figure called "The Patriarch."

At the beginning of the novel, the Sharers encounter a non-Sharer community from another planet, which threatens them. The Sharer Merwen the Impatient realizes that they must find out whether other kinds of "people" can share their life or not. Merwen goes to Valedon, to recruit a young Valan, Spinel, to return to Shora and attempt to learn their ways. This venture leads to disagreement within the Sharer community, despite the fact that a Valan noblewoman, Lady Berenice, who takes the Sharer name, Nisi the Deceiver, has lived amongst the Sharers before. Berenice, daughter of the first traders to settle on Shora, spent her childhood among the Sharers and returned to Valedon to spend time with her fiancé, Realgar, a Sardish Valan general.

To Spinel's initial distress, after some time on Shora, his skin takes on a purple hue as his body becomes acclimated to the breathmicrobes that allow Sharers to stay longer underwater. He gradually learns the Sharer way, as a man and is finally accepted by Merwen's daughter, Lystra, who risks her life to save him from a Shockwraith, a deadly underwater creature. Lystra's former 'lovesharer', Rilwen, is "stonesick", meaning that she has developed an addictive fascination with the stones and gems the Valan traders bring to Shora. After Rilwen's death, Lystra and Spinel become lovesharers.

Realgar and Malachite, an envoy of the Patriarch, visit Shora to place it under Patriarchal rule. After this visit, Realgar, Berenice, and Spinel return to Valedon. Berenice eventually realizes Realgar has no intention of letting her return to Shora and that the Patriarch has arranged for Shora to come under patriarchal rule rather than exist self-governed. She fakes her death and sneaks away to Shora, despite the fact that travel has become forbidden. Meanwhile, Spinel returns to his family, who are living under Dolomite rule after the siege of their city. He helps the citizens stage a peaceful act of rebellion against the Dolomites and becomes a "spirit caller." When Spinel hears what has happened to Shora while he has been away, he returns to the capital of Valedon to try and gain passage back to Shora.

Realgar, still grieving the death of Berenice but obeying orders, returns to Shora to force the Sharers to obey Patriarchal rule and Valan law. The Sharers peacefully continue their way of life, despite the increasing military presence and escalating violence of Realgar's soldiers. Realgar has several Sharers kidnapped and probed for information, though the Sharers enter "whitetrance", a vulnerable state close to death in which their bodies turn wholly white as their minds retreat within. Some kidnapped Sharers, including Lystra, are taken to Valedon. Spinel sees Lystra and calls out to her, breaking her whitetrance. Spinel's ability to reach the Sharers causes Realgar to take him and the Sharers back to Shora, where the Sharers are tortured. Several of Realgar's soldiers become increasingly sympathetic to the Sharers. Among them is the scientist, Siderite, who is tasked with learning the Sharer's advanced science for the Patriarch and becomes apprenticed to Usha, a "lifesharer" who practices advanced medicine.

After finding out Berenice still lives, Realgar publicly executes several Sharers, and almost executes Merwen before taking her back to the military compound. Merwen attempts to "share" with Realgar to cure him of what the Sharers consider the Valan's sickness, the fear that causes them to "hasten death" (kill). The Sharers continue to protest through non-violence, though this results in many Sharer deaths. They discover a cure for the microbes the Valans have released to keep away the dangerous seaswallowers and counteract it to allow the seaswallowers to return and bring the planet back into harmony. The return of the seaswallowers nearly destroys the Valan military compounds, forcing the Valan troops to withdraw. Travel between the two planets becomes banned. Berenice remains on Shora, while Spinel almost returns to Valedon to spread nonviolent resistance against the Patriarch. However, when Lystra decides not to follow, Spinel decides to stay as well.

Slonczewski's Sharers are committed to environmental sustainability. They restrict their own reproduction to sustainable levels and view all species on their world as interconnected, including seaswallowers, a dangerous species that often destroy the rafts on which Sharers live and whose natural actions result in the deaths of Sharers. The Sharer worldview extends to their environment, their surrounding ecosystem. They cannot act upon their plants and animals without being acted upon in return. So, for example, because Sharers consume plants and animals as food, they accept the fact that they in turn will become food for other life forms; that predators will ultimately consume them.