Lawyers for David Turner had argued the doll was not covered by a law banning the importation of obscene items.
But a judge dismissed that, saying "any right-thinking person" would find the doll obscene.
Turner, 72, of Ramsgate, Kent, had already admitted possessing more than 34,000 indecent images of children. He will be sentenced in September.
Other men have been convicted for importing child sex dolls, but this was the first case where the question of whether a doll is indecent or obscene had been tested by the courts.
The National Crime Agency (NCA) said it was "the first ruling of its kind".
Under the 1979 Customs and Excise Management Act it is against the law to import obscene or indecent items.
NCA officers arrested Turner - an ex-church warden - in November last year after Border Force officers intercepted a 3ft doll that he was attempting to import from China.
He claimed that it was intended to be a "companion" for him and his wife.
Investigators said Turner already had two other child dolls at his home, one of which they claimed he had "sex" with.
The charge was brought over a 3ft 10in sex doll already in his possession, which he bought clothes for.
In an earlier hearing, Turner also admitted possessing more than 34,000 indecent images of children, aged from three to 16, contained on 17 pen drives.  Of the images, 138 were of the most serious kind, Category A.
When interviewed, he also admitted "secretly" taking photographs of girls aged six to 11 in public places.
Judge Simon James said the importation of a child sex doll was an "unusual offence" and that it "adds a degree of complexity" to the sentencing.
Turner was freed on bail and will be sentenced on 8 September. The maximum sentence is seven years.
By Danny Shaw, BBC home affairs correspondent
More than 100 child sex dolls have been seized in the UK as part of a special operation set up by the National Crime Agency (NCA) and Border Force.
Investigators believe the dolls, which are made to be as life-like as possible, are being imported by people who have a "sexual interest" in children.
Advertised as "adult dolls" on sites including EBay and Amazon, they are manufactured in the Far East and sent to Britain via courier, fast parcel and other delivery services.
Hazel Stewart, operations manager for the NCA, said when people have been questioned by police about the dolls some have claimed they were bought for a "joke", others say they for "companionship".
"If it was a Marks and Spencer mannequin I wouldn't be stood here," she said.
Read more from Danny Shaw.
Hazel Stewart, from the National Crime Agency, said: "We know [child sex doll] purchases can indicate other offences against children, as was the case against Turner who had a sickening stash of abuse images.
"Importers of such obscene items should expect to have law enforcement closing in on them."
Jon Brown, head of development for the NSPCC, welcomed the ruling, but wants more to be done to tackle the issue of child sex dolls.
"The NSPCC is calling on government to take action to criminalise the manufacturing, distribution and possession of these grotesque dolls, in the same way it does indecent images of children," he said.
The NSPCC has previously called for sites selling the dolls to take them down, telling BBC Radio 5 live that it could lead into "contact offending" and "normalises" it.
A judge has ruled that a child sex doll imported by a former primary school governor is an obscene item.