The phrase "although decidedly" is correct and usable in written English.
It can be used to introduce a contrast or concession, emphasizing a strong opinion or conclusion despite potential opposing views.
Example: "Although decidedly challenging, the project proved to be incredibly rewarding in the end."
Alternatives: "even though clearly" or "despite being unmistakably".
Although decidedly left-wing, Dissent critiques both liberal and conservative views and occasionally reserves its strongest criticism for the left.
Although decidedly muskier, Wolfgang's Steakhouse takes pains to put more than a bit of surf on its turf.
Although decidedly a man of peace, Alexander became the reluctant champion of the oppressed Slav peoples and in 1877 finally declared war on Turkey.
Although decidedly low-concept (not a Disney character in sight), several of the rides — notably the pirate ship — deliver the all-important stomach-inside-out sensation.
Kimchi has become so expensive that some restaurants in the capital no longer offer it free as a banchan, or side dish, a situation akin to having an American burger joint charge for ketchup, although decidedly more calamitous here.
But Jonathan Lippman, the chief judge of the Court of Appeals, wrote in an opinion that the fraud, "although decidedly criminal in nature," did not amount to an unlawful taking of property from New York City Transit because the transit agency never actually possessed the fare that it would have otherwise been paid.
The master bedroom is similarly spare, but the children's rooms are a bit more whimsical and colorful, although still decidedly modern: it isn't every child's room that has its own Saarinen or Bertoia furniture.
Although a decidedly different species, today's polar bears have Irish brown bear genetic material in their cells, indicating that the polar bear "Eve" was brown and that bear evolution was far from simple.
Although he is decidedly liberal in his own leanings, he is apolitical in his targets.
Although there are decidedly luxurious places to stay (the Ritz-Carlton Powerscourt, for example, with its Gordon Ramsay restaurant), they are not the only choice.
And although it was decidedly unmentioned today, Mr. Clinton is nearing the final stages of a disciplinary hearing before state authorities that could result in the suspension or revocation of his license to practice law.
