Officers were alerted to the find just south of Porth Neigwl (Hell's Mouth), near Abersoch on the Llyn Peninsula, at about 16:00 GMT on Sunday.
Police and members of the coastguard placed a 100m (330ft) cordon around the object.
The Ministry of Defence confirmed it was possibly some kind of ordnance.
Explosive experts are set to detonate the device on Monday morning to make it safe.

On Thursday, the firm acknowledged it read the anonymous blogger's emails in order to identify an employee it suspected of leaking information.
Microsoft owns Hotmail, a free email service now called Outlook.com.
John Frank, deputy general counsel for Microsoft, said it took "extraordinary actions in this case".
While the search was technically legal, he added Microsoft would consult outside counsel in the future.
Microsoft's actions came to light this week as part of a legal case by US prosecutors against an ex-Microsoft employee, Alex Kibalko, who was a Russian native based in the company's Lebanon office.
In 2012, Microsoft had been alerted to the fact that the blogger, whose identity was kept anonymous in the court papers, had been given some stolen lines of code from the not-yet-released Windows 8 operating system.
The blogger then posted screenshots of the unreleased Windows operating system to his blog.
To figure out the source of the leak, Microsoft began an investigation and, as part of that search, looked into the blogger's accounts to find out the name of the employee.
The search was legal because it fell within Microsoft's terms of service which state that the company can access information in accounts that are stored on its "Communication Services", which includes email, chat areas, forums, and other communication facilities.
The terms of service add: "Microsoft reserves the right to review materials posted to the Communication Services and to remove any materials in its sole discretion."
Nonetheless, revelations of the search have led to renewed focus on the privacy violations of technology firms.
It has also left Microsoft in a difficult position, as the firm has often criticised rival Google for its automatic scanning of users' emails in order to serve them with advertising.

Both men retired on 105 as they punished a tiring students attack.
Donald, 19, is the second youngest player to reach three figures for Glamorgan.
The record is still held by Mike Llewellyn, who was aged 18 when he scored a century against Cambridge University in 1972.
Donald had previously made 98 in the final game of the 2015 season against Gloucestershire.
"It was nice to tick it off, it's been looming over me a bit during the winter.  It's really special, nice to get over the line in the first game of the season," Donald told BBC Wales Sport.
"Pleased for Dai as well, to have two maiden first-class tons in one game was a pretty good effort."
Lloyd, whose previous best was 92 at Northampton in 2015, reached his century with a six.
"I was just pleased to get over the line, and for Aneurin as well," he said.
"Last season was a stepping stone for me and hopefully I can go one step further with bat and ball."
The pair's run-scoring feats are likely to provide a difficult selection decision for Glamorgan's Championship opener against Leicestershire, with South Africa international batsman Colin Ingram yet to come into the side.
"Hopefully we've put ourselves up there for selection. It's about being in the mix, and you can't ask any more of ourselves to both get hundreds," said Donald.
"We accept this year there's going to be more competition for places which is a great thing for the club, pushing us as batters and bowlers."
Donald could still beat Matthew Maynard's record for the youngest Championship century, achieved in 1985.
Earlier Glamorgan's top order had all spent time at the wicket without going on to make big scores, as James Turpin and Alex Thomson claimed two wickets apiece.
The run-scoring exploits came after day one of the three-day match was washed out.
Part of a Gwynedd beach has been closed after a member of the public found a suspected explosive device, North Wales Police has said.
[NEXT_CONCEPT]
Microsoft is caught up in a privacy storm after it admitted it read the Hotmail inbox of a blogger while pursuing a software leak investigation.
[NEXT_CONCEPT]
Glamorgan batsmen Aneurin Donald and David Lloyd both hit maiden first-class centuries as they piled up 444-7 against Cardiff MCCU.