Tests have shown rat poison killed the female bird of prey, which was discovered in a quarry in Penmaenmawr in June.
North Wales Police said this was the second suspicious death of a peregrine falcon in the last few months.
The protected bird is a threat to racing pigeons and officers are appealing to the community for help.
"It is very sad that a highly protected and rare bird is being wiped out of north Wales because someone is laying poison indiscriminately for their own gain," said Sgt Rob Taylor.
"We are aware of the reasons and methods of poisoning and also the locations of our birds, so we will be working closely with the RSPB and other voluntary societies to put a stop to this practice and catch the offender."
Last month, four peregrine falcons were found dead in their remote nest at a quarry in Gwynedd.
One adult and three chicks were discovered in the nest at Dyffryn Nantlle and police suspected foul play.

The plane came down en route to Russia from the resort of Sharm el-Sheikh.
A group linked to so-called Islamic State (IS) said it bombed the plane. Nearly all the victims were Russians.
Russia has said a bomb brought down the Metrojet Airbus, after finding what it said were "traces of foreign explosives" on the debris.
It has vowed to "find and punish" the perpetrators. In response to Monday's findings, Kremlin spokesman Dmitry Peskov re-iterated that "our experts concluded this was a terrorist attack".
Russian plane crash: What we know
Russia suspended all flights to Egypt in the wake of the attack, with the UK also suspending flights to and from Sharm el-Sheikh.
However, the head of the Egyptian committee investigating the crash, Ayman al-Muqaddim, was quoted by state TV on Monday as saying there was "no evidence that there is an act of terror or illegal intervention".
IS-affiliate Sinai Province said it had destroyed the plane because of Russian air strikes on Syria.
Last month IS's magazine published a photo of what it claimed was the improvised bomb that brought down the airliner.
The picture in Dabiq showed a Schweppes Gold soda can and what appears to be a detonator and a switch.
The crash has dealt another blow to Egypt's vital tourism industry, already struggling after years of unrest. Egypt's tourism minister told Reuters earlier this month that tourism revenues for 2015 will be at least 10% below last year's.

A Department for International Trade report shows 5,443 new posts were created in 2015-16 - a 7% increase on the previous financial year.
Almost 7,000 jobs were created or safeguarded by foreign direct investment during the same period.
The report said the UK remains the most popular EU destination for overseas firms.
Welsh Secretary Alun Cairns said Wales had a "tremendous platform" to build on as the UK prepared to leave the EU.
He said: "These figures demonstrate that Wales remains an attractive destination for overseas investment, providing a valuable boost to the economy and creating thousands of jobs.
"Our country now has a tremendous platform to build on over the coming months as we make plans to leave the EU, forge stronger relations with international partners and attract more investment from emerging markets."
The figures come as First Minister Carwyn Jones said the referendum result meant it was time to sell Wales to the world like "never before".
He has appointed officials to represent Wales in EU negotiations and reiterated his support for the UK to remain in the single market at a press conference to mark 100 days since he was reinstated as first minister.
A peregrine falcon found dead in Gwynedd was poisoned, police have confirmed.
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Egyptian investigators say they have so far found no evidence that terrorism caused a Russian jet to crash in the Sinai in October, killing 224 people.
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More than 5,000 new jobs were created in Wales as foreign investments in the UK hit a high, government figures show.