Moominpappa at Sea (Swedish: Pappan och havet, literally "The Father and the Sea") is the eighth book in the Moomin books by Finnish author Tove Jansson. First published in 1965, the novel is set contemporaneously with Moominvalley in November (1970), and is the final installment in the series where the titular Moomin family are present within the narrative.

The book follows the titular Moomin family as they arrive on a deserted island in the middle of the sea, documenting their experiences as they become familiar with their newfound surroundings. The book is notable for its introspective and contemplative tone, which is in contrast to the other books found within the Moomin series.

The book is based primarily around the character of Moominpappa, who was heavily inspired by Jansson's father, Viktor Jansson. The book's original title is a loose reference to Ernest Hemingway novel The Old Man and the Sea, though this is not reflected in the translation.

Moominpappa becomes dissatisfied with his life in Moominvalley, so he organizes a journey for his family to find a lighthouse in the sea. Upon arrival, they discover the island to be a desolate and lonely place, inhabited only by an unfriendly fisherman.

Moominpappa wishes to become the lighthouse keeper, but he gives up when he cannot figure out how to fix the lantern. He tries other projects, such as building a pier, fishing, and researching a small lake, but fails at all of them.

Meanwhile, Moominmamma becomes increasingly melancholic as her dream of planting a garden fails, and her longing for home grows stronger. She eventually starts painting the walls of the lighthouse with flowers until she has painted all of Moominvalley. She discovers that she can walk inside the painting to find peace.

Moomintroll explores the nearby woods and finds a thicket that he eventually moves into. He is disappointed to find it already inhabited by ants and asks Little My for help in getting them to move elsewhere. Little My solves the problem by exterminating the ants with petroleum, much to Moomintroll's dismay.

At night, Moomintroll looks for the seafillies whom he admires greatly. The fillies are selfish and mean to Moomintroll, but he does not care. As he tries to attract them by waving his lamp, he instead attracts the Groke. Every night, Moomintroll tries to call for the seafillies, but only ends up being accompanied by the Groke. Slowly, he starts to grow fond of her, and when the lamp ultimately runs out of kerosene, the Groke is no longer cold.

As the story draws to a close, the once disjointed family slowly comes together again. Together, they confront the sea and save the lonely fisherman. When they discover that his birthday is coming, they invite him to a party at the lighthouse, which he reluctantly attends, only to slowly realize that he is the original lighthouse keeper. Moominpappa and the fisherman find new purpose in their lives through their shared experience as the novel draws to a close.

The map at the front of Moominpappa at Sea locates the island in The Gulf of Finland. It even gives a location: 60°7′12″N 25°45′50″E / 60.12000°N 25.76389°E / 60.12000; 25.76389. This is about 42 kilometres (26 mi) east of Helsinki, and 11 kilometres (7 mi) south of the coast of Finland; though no island exists there in real life. Jansson is said to have used real-life Söderskär Lighthouse in Porvoo as a model when writing the lighthouse island of the book.

The novel has been adapted into animation several times, including episodes 25 and 26 of the 1990 television series Moomin, and episodes 23, 24, and 25 of the 2019 television series Moominvalley.

After Tove Jansson's death, the producer of the 1990 Moomin television series, Dennis Livson, planned to make a film adaptation based on Moominpappa at Sea, but Tove Jansson's niece Sophia Jansson did not give him permission for the adaptation.

Kirkus Reviews wrote, "A psychological drama with an allegorical level, this chapter of the Moomin story cedes Moomin territory to the adults without totally dispossessing the children."

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