NATO spokesman James Appathurai was quoted as describing the Slovene move as "a good development".
"We are confident that both Albania and Croatia will be able to take their full place in NATO at the summit, which is something we are of course all as the NATO family very much looking forward to," the spokesman said.
Earlier on Friday, Slovene Parliament Speaker Pavel Gantar signed the law ratifying the accession document, followed by President Danilo Tuerk.
The law on the ratification is to be published in Slovenia's official gazette in the course of the day, which will mark its official enactment.
Slovenia's parliament ratified the protocol in February but a nonparliamentary party contested the ratification and wanted to hold a referendum on the matter. However, it did not manage to collect the required number of 40,000 signatures within a 35-day term which expired on Thursday.
The party, which Reuters described as "a small nationalist party", cited Croatia's border dispute with Slovenia as the reason for the referendum. Reuters recalled that Slovenia "has blocked Zagreb's European Union entry talks" over the same issue.
The other 25 NATO member states have already ratified Croatia's accession protocol.