British, French, German, Austrian and Italian sites were down for about 30 minutes on Sunday during a peak pre-Christmas shopping period.
A group of cyber activists, Anonymous, is hitting firms that withdrew services from whistle-blowing site Wikileaks.
There has been no official comment from Amazon or from Anonymous.
Amazon sites ending .it, .de, .uk, .fr and .at - which are all hosted in Dublin - were unavailable for about half an hour at about 2115 GMT on Sunday, according to a Twitter posting by web monitoring firm Netcraft.
However, all servers are now back up and running after a brief delay, the firm says.
There were no clear indications that Anonymous was responsible for the disruption to Amazon, and the group has not said it carried out an attack.
However, it did plan to mount a distributed denial-of-service (DDos) attack on Amazon several days ago.
Supended accounts On Friday Anonymous publicly abandoned plans to hit Amazon, saying they did not have the "forces".
"We cannot attack Amazon, currently. The previous schedule was to do so, but we don't have enough forces," read one message on Twitter on Friday.
DDoS attacks, which are illegal in the UK, involve overloading a website with high numbers of requests so it stops working.
Several Twitter accounts attributed to Anonymous and its campaign have been suspended over the attacks.
The group's Operation Payback Campaign has also targeted the websites of Paypal, Mastercard and Visa, as well as the Swedish Prosecutor's website after a case was brought there against Wikileaks founder Julian Assange.
Mr Assange is in British custody awaiting an extradition hearing after being accused of sex crimes in Sweden. He denies the charges.
Wikileaks has angered the US government by publishing large caches of secret documents online, including US diplomatic cables.
The government has written to Wikileaks, saying it believes its actions are illegal, but it has denied putting pressure on firms such as PayPal to withdraw services.
WikiLeaks has refused to link itself with Anonymous, saying "we neither condemn nor applaud these attacks".