"It is with great sadness that I have received the news of our dear friend, King Harald's passing," he said in a statement.
King Harald was deeply cherished by Norwegians throughout his 35-year reign.
He ascended the throne on January 17th, 1991 following the death of his father, King Olav V, ruling Norway for over three decades under the royal motto "Alt for Norge" (All for Norway).
"King Harald’s life story is exceptional. After enduring the hardships of war and exile in his childhood, he was able to return with his family to a free Norway and witness the nation’s remarkable development throughout the twentieth century, before succeeding his father as King of Norway," said King Carl XVI Gustaf.
The Scandinavian royal families are close, and the Swedish king is the godfather of Haakon, King Harald's eldest son, who will now succeed him.
"Throughout the years, King Harald has been a good friend to me, my family, and to Sweden. King Harald is now mourned by many in our country," said King Carl XVI Gustaf, extending his condolences to Queen Sonja.
"I wish my dear Godson Haakon, who now succeeds his father as Head of State, success and good fortune in the important role that lies ahead of him, in service to our brother nation and its people."
The Swedish government said it would be flying the flag at half mast on Friday.