A survey by the charity found the majority of Scottish local authorities are not paying their foster carers a sufficient allowance to cover the costs of looking after a child.
Two thirds of Scotland’s 32 councils paid foster carers less than the £130 a week minimum the Fostering Network recommended in 2011-12.
In some authorities foster carers were given as little as £72 a week, compared to £156 a week in the most generous Scottish councils.
The Scottish government recommends councils pay the Fostering Network’s rates but, unlike the rest of the UK, it does not enforce this, leaving councils free to set their own rates.
This means foster children in Scotland are among the worst off in the UK, according to Sara Lurie, director of the Fostering Network in Scotland.
She said: “There is no reason why Scotland is lagging behind when it comes to ensuring their needs are met and foster carers have their costs covered.
“No foster carer should have to pay out of their own pocket to make sure fostered children get the essentials they need.
“The Scottish government must act now and introduce statutory minimum allowances for fostered children."
The charity also found that 17 local authorities in Scotland did not increase the amount paid to foster carers in 2011-12 from the level given in the previous year.