The largest U.S. drugmaker is paying a hefty premium of 62% to get access to Array's approved treatments for skin cancer as well as its other experimental drugs.
Under new Chief Executive Officer Albert Bourla, Pfizer has been exploring ways to diversify its pipeline as its blockbuster pain drug Lyrica loses patent protection.
The company has also been pushing its "15 in 5" plan to launch 15 experimental treatments, each with at least $1 billion annual sales potential, over a five-year period and has been investing in cancer drugs and gene therapies.
In a conference call in April, Pfizer said it was considering "bolt-on" deals worth a few billion dollars to complement its pipeline.
Pfizer is paying $48 per Array share, which rose 60% to $47.38 in light premarket trading. Pfizer's shares were marginally higher.
The U.S. Food and Drug Administration last year approved Array's oral combination treatment for use in patients with melanoma - the deadliest form of skin cancer.
The company is also testing its triple combo therapy in colorectal cancer patients.
"(The acquisition) sets the stage to create a potentially industry-leading franchise for colorectal cancer alongside Pfizer's existing expertise in breast and prostate cancers," Bourla said.
Pfizer said it expects to complete the deal in the second half of 2019.
The transaction is expected to add to earnings beginning 2022, and will reduce adjusted earnings per share by between 4 and 5 cents this year and in 2020, Pfizer said.
Pfizer said it expects to finance the majority of the deal, which has an enterprise value of about $11.4 billion, with debt and the remaining with existing cash.