Takeda — which in September revealed it was scouting for multibillion-dollar acquisitions to reduce its dependence on domestic sales — agreed to pay $24 for each Ariad share, a premium of about 75 percent to its Friday close.
Ariad stock was up 73 percent at $23.77 in morning trading on Monday. In 2016, its shares had nearly doubled.
Takeda's top-selling blood cancer drug Velcade is expected to face generic competition this year and other key products go off patent from 2020.
Cancer drugs are appealing to large drugmakers, with high prices being paid for promising assets. San Francisco-based Medivation was bought by Pfizer for $14 billion in August.
Given the scarcity of commercial-stage oncology assets and significant potential synergies, other Ariad bidders could emerge, SunTrust Robinson analysts said.
In November, Reuters reported that Takeda's negotiations to acquire Valeant's Salix stomach-drug business had stalled, citing sources.
With the Ariad deal, the Japanese drug giant gains access to the leukemia drug, Iclusig, which is expected to generate sales of $170 million to $180 million in 2016.
Ariad came under fire in October for price increases of Iclusig.
A U.S. Food and Drug Administration decision on its lung cancer treatment, brigatinib, which is being touted as a potential blockbuster, is expected by April.
Sarissa Capital Management, a hedge fund run by investor Carl Icahn's former healthcare lieutenant, had announced a stake in Ariad in 2013.
Sarissa will tender its shares to Takeda, if the deal goes through.
Ariad suspended Iclusig sales in 2013 after data showed it was associated with serious complications, but resumed marketing after the FDA sanctioned its use in a narrower patient population.
The equity value of the deal, which is expected to add to Takeda's earnings in 2018, is $4.66 billion, according to Reuters calculations. Takeda plans to fund the transaction by taking on $4 billion in new debt.
Evercore Partners is Takeda's financial adviser, while Cleary Gottlieb Steen & Hamilton are providing legal counsel. Ariad's financial advisers include J.P. Morgan, Goldman Sachs and Lazard, while Paul, Weiss, Rifkind, Wharton & Garrison is its legal counsel.
Takeda's last major deal was in 2011 when it paid nearly $14 billion for Swiss drugmaker Nycomed.