Justina Kristina Casagli, née Wässelius, (4 October 1794 – 1841), was a Swedish opera singer. She was internationally famous and active in Italy and Germany. She was a member of the Royal Swedish Academy of Music.
Born in Stockholm, Sweden, the child of a tapestry manufacturer, Justina was enlisted at Dramatens elevskolain 1805, following the example of her older sister Jeanette Wässelius, and was from this date active at the stage under the tutorage of Sofia Lovisa Gråå. She was contracted at the Royal Swedish Opera, where her older sister was prima donna, in 1812. The same year, she married the Italian dancer Luigi Casagli, and is thus known as Justina Casagli.
In Sweden, she was noted for her appearance, grace and flexible voice. Her sister Jeanette Wässelius spent her entire career in Sweden and became the great opera prima donna of her generation on the Swedish opera, known as Wässelia. Justina Casagli was elected to the Royal Academy of Music with her sister Jeanette and Anna Sofia Sevelin in 1817.
In 1818, she left Sweden with her husband and travelled to Italy, where she made a huge success in Rossini"s La Cenerentola in Turin. She continued her triumph - it is said she "made herself immortal" - on the stages of Rome in Rossini's La donna del lago in 1823 and in Lucca in 1827 in Cimarosa's Il matrimonio segreto, before she was employed in the National Theatre Munich in Münich in Germany.
After the death of her husband in 1831, she tried to find employment at the opera in Stockholm, but was denied a position, as her place there had been filled by Henriette Widerberg, and there were concerns that her voice had been damaged. She spent her last years in Parma. She committed suicide in 1841 by throwing herself out a window during a depression caused by her financial difficulties. She died in Parma, Italy.
- Herman Hofberg: Svenskt biografiskt handlexikon (Swedish biographical dictionary) (1906) (Swedish)
- Arvid Ahnfelt: Europas konstnärer (The artists of Europe) (Swedish)
-  Herman Hofberg: Svenskt biografiskt handlexikon (Swedish biographical dictionary) (1906) (Swedish)
-  Arvid Ahnfelt: Europas konstnärer (The artists of Europe) (Swedish)
- Nils Bohman: "Svenska män och kvinnor. Bok 2" (Swedish Men and women. Dictionary. Book 2) (Swedish)
- Gustaf Hilleström: Kungl. Musikaliska Akademien, Matrikel 1771-1971 (The Royal Academy of Music 1771-1971) (in Swedish)